Differentiated Implementation of Controls: The Internal Border Regimes of Schengen

The use of temporary internal border controls in the Schengen Area reached a new record during the outbreak and spread of the Covid‐19 pandemic. Several member states chose to introduce internal border controls that had not done so up until that point, while others have had continuous border controls in place since the refugee crisis in 2015. Other member states have never or only rarely used this temporary opt‐out from the principle of free movement of persons inside Schengen. This development has raised the question of whether we are moving towards the disintegration of the Schengen Area as member states make very different choices regarding controls towards their EU neighbours. Comparing the use of internal border controls by all member states, the article suggests the concept of differentiated implementation to explain the variations in internal border regimes among Schengen member states. Focusing on two dimensions of control, the control of movement originating internally or externally to the EU, a typology is developed that conceptualises differentiated implementation as four types of internal border regimes. The analysis illustrates these four types by applying them to the use of controls up until 2022, identifying the grouping of member states. The proposed typology of internal border regimes presented represents an ambition to conceptualise the differences in internal border control use that have previously often been understood as a general problem of the Schengen regime.


Introduction
In the Schengen Area, the use of temporary internal border controls (IBCs) reached a new record during the outbreak and spread of the Covid-19 pandemic. Following developments in March 2020, many EU member states chose to introduce IBCs that had not done so up until that point. Previous research has suggested that the expanded use of IBCs in 2020 was a "knee-jerk reaction" made available as a result of the "taboo" of using IBCs in the EU being broken during the refugee crisis of 2015 when many member states introduced IBCs and then continued to prolong them far beyond their intended short-term use (Wolff et al., 2020). Yet some member states that had previously introduced IBCs chose not to do so due to the Covid-19 crisis. As member states thus make very different choices about controls towards their EU neighbours, this development has raised the question of whether we are moving towards the disintegration of the Schengen Area. How should the highly diverging practices of how member states make use of IBCs be conceptualised? As the use of temporary IBCs becomes more widespread, differences in their use among EU members has the potential to create more long-term differences in the management of internal borders, and thus affect the legitimacy and status of the principle of free movement in Schengen. This article proposes that the concept of differentiated implementation can be used to describe this process.
Since the first expansion of IBC use in 2015, there has been extensive research on their development over time (Gülzau, 2021) and on the motives and justifications of controls (Pettersson Fürst, 2023;Votoupalová, 2019) as well as the legal implications and responses from the EU institution to these controls (Mantu, 2021). Several authors have also studied the impact of controls in specific contexts (Barbero, 2020;Casella Colombeau, 2020;Evrard et al., 2018), on citizen attitudes (Lutz & Karstens, 2021), or the use of controls by a specific group of member states (Karamanidou & Kasparek, 2020). Recently, progress has been made towards synthesising the empirical analysis of these controls into broader analytical frameworks for understanding European border governance (Schimmelfennig, 2021) and specifically towards the internal border regime, focusing on the consequences of three categories of reasons for controlmigration, terrorism, and Covid-19 (Guild, 2021).
The present article contributes to these ongoing efforts by proposing a typology grounded on a distinction between two types of border functions in the Schengen Area: the regulation of internal and external movement, respectively. Drawing on previous conceptualisations of border regimes, the article develops and proposes a typology of differentiated implementation, which can be used to identify the kind of border regime that is being created at the internal borders of the EU. Using the member states notifications of IBC introductions from September 2015 (following the "refugee crisis") until the end of 2022, the article's empirical analysis illustrates how the typology can be used to identify core differences between groups of member states' implementation of the Schengen border rules.
By providing a typology that enables analytical differentiation between different ways in which member states employ IBCs, this article contributes a more nuanced understanding of what these controls mean. This matters as a foundation for further empirical studies of member state practices, but it can also be useful for developing arguments about whether we should view the use of IBCs as fundamentally in line with, or opposed to, the principles of the Schengen agreement.

Background: The Rules of Temporary Internal Border Controls in Schengen
The general principle of the Schengen agreement is that its member states have agreed to abolish border checks between them to guarantee the free movement of persons, which enables its citizens to move freely in the Schengen Area for purposes of leisure, work, and living. In principle, this freedom of movement is supposed to happen without being subject to border control when crossing a Schengen border (referred to as an internal border). The countries in the Schengen Area have agreed on a number of common rules to enable the principle of free movement, including harmonisation of visa rules, cross-border police cooperation, and rules for external border controls. The rules governing the Schengen Area borders were laid out in the Schengen Borders Code (SBC; Regulation of 15 March 2006, with some amendments adopted in 2013, and are in their current form regulated by the SBC adopted in 2016 (Regulation of 9 March 2016. The rules governing the use of so-called temporary IBCs can be found in articles 25-35 in the 2016 SBC (articles 23-31 in the 2006 SBC). Article 25 reads: Where, in the area without internal border control, there is a serious threat to public policy or internal security in a member state, that member state may exceptionally reintroduce border control at all or specific parts of its internal borders for a limited period of up to 30 days or for the foreseeable duration of the serious threat if its duration exceeds 30 days. (Regulation of 9 March 2016 IBCs can be introduced on a few different grounds, e.g., for foreseeable events (Regulation of 9 March 2016, 2016, art. 25), for events requiring immediate action (Regulation of 9 March 2016, and in exceptional circumstances, where the overall functioning of the Schengen Area is at risk (Regulation of 9 March 2016, 2016, art. 29). As has been noted by previous research, which article has been used as grounds for the introduction of controls has been inconsistent. During and after the refugee crisis, member states have used different articles and/or shifted which article of the SBC they base their introduction on, and during the Covid-19 crisis, member states' "flexibility in appealing to articles 23, 25, and 28 while conducting similar checks illustrates the ambiguity of the legal provisions on internal border controls" (Heinikoski, 2020, p. 6). This shows the room for different interpretations and thus implementation that is available in the SBC for when IBCs can be deemed an appropriate measure.
IBCs work in two ways to disrupt cross-border movement. First, subjecting people to controls at the border causes delays in travel times and requires people to carry necessary identification documents, which in itself is a disruption to the principle of freedom of movement. Second, in being controlled, people may be denied entry based on not having appropriate documents, or by having appropriate documents but belonging to a category of people not allowed entry, which effectively means denial of free movement. Thus, the more extensive the categories denied entry are, and the more extensive the controls are in scope and time, the more disruptive internal controls are to the principle of free movement.
Importantly, the rules from 2016 emphasise that controls are only to be used as a "last resort" (Regulation of 9 March 2016, 2016, art. 25.2) and specify that prolongations should not exceed six months, or in "exceptional circumstances," the "total period may be extended to a maximum length of two years" (Regulation of 9 March 2016March , 2016. This is in contrast to the original rules that specified 30 days plus a maximum 30-day prolongation as the rule. Despite these changes that allow for longer prolongations, several countries have continued to prolong their IBCs for recurring six-month periods since 2015, vastly exceeding the two-year maximum limit. How the prolongation of controls beyond the two-year maximum should be understood is somewhat unclear: Heinikoski (2020, p. 5) argues that by combining different legal bases, "member states have had much leeway" in prolonging their controls, "ignoring the maximum periods outlined in the rules," and the Court of Justice of the European Union (2022) has declared prolongations incompatible with the Schengen acquis. Yet, while the member states "fall short of demonstrating necessity and proportionality" required by the SBC in their justifications of controls, the Commission has not called for a termination of controls, nor used all the monitoring tools at its disposal (Montaldo, 2020, pp. 528-529).
In 2021, the Commission presented a new legislative proposal (European Commission, 2021) for an amended SBC, which has yet to be adopted. This new proposal seeks to address the problem of IBCs disrupting the principle of free movement by, inter alia, making reporting and motivation demands clearer when prolonging controls and by encouraging member states to introduce new measures to be used instead of IBCs, including more "regular police checks in border zones." However, this proposal does not bring an end to IBCs, as it makes clear that the use of IBCs is within the rights of the member states, and it also establishes the two-year prolongation time frame as a standard maximum with provisions for continued controls beyond that time frame. Thus, although the proposed amendment to the SBC does aim to improve coherence in the implementation of rules, it does not solve the inherent tension between free movement and the use of IBCs.

Previous Research: Border Controls and Disintegration in the Schengen Area
In the literature on differentiation in the EU, Schengen is typically understood as a "paradigmatic case" of differentiation (Leuffen et al., 2022, p. 338) as some EU members remain outside Schengen (Ireland, Romania, Bulgaria, Cyprus) while several non-EU members are part of the Schengen Area (Norway, Iceland, Switzerland, Lichtenstein). Integration in the Area of Freedom, Security and Justice, which includes the border policies of Schengen, has increased considerably over time (Leuffen et al., 2022, pp. 338-340) but has been challenged by the internal re-bordering crises. Schengen thus presents a case of EU differentiation in itself, which has been analysed extensively by previous research. However, the focus of this article is the differences internal to the Schengen Area, whose members belong to the same formal border regime and have common rules regulating the control of internal and external borders. All are thus part of the Schengen regime, but make different use of its provisions for internal controls.
Already in the early 2000s, Groenendijk (2004) found that the use of IBCs varied considerably between mem-ber states, but, at this time, the majority of controls were used for temporary events such as high-level meetings. Controls at the internal borders had been an issue before 2015, for example in 2011 following the "Italo-French row" (Zaiotti, 2013). However, it was during the refugee crisis of 2015 and the prolongations of controls that followed that more widespread concern was raised about a "crisis of Schengen" and what it would mean for European integration. Several authors raised the question of what consequences a more everyday use of IBCs will have for the principle of freedom of movement in the Schengen Area and their potential to undermine this core principle of European cooperation. Initial observations of re-bordering during the refugee crisis talked of the "death of Schengen" and the use of IBCs as the beginning of the disintegration of the European Union (Brekke & Staver, 2018). IBCs were argued to undermine the Schengen principle of free movement and the continuous prolongations by several countries have been criticised for shifting the meaning of what constitutes a threat (Karamanidou & Kasparek, 2020). In the most recent re-bordering crisis, the Covid-19 outbreak, the use of IBCs has been further criticised, as they caused unprecedented restrictions to free movement in the Schengen Area countries (Montaldo, 2020) and were implemented in an uncoordinated and unpredictable manner (De Somer et al., 2020;Heinikoski, 2020;Thym & Bornemann, 2020). In many member states, IBCs were also combined with other restrictions to movement, both internal (such as national or regional movement restrictions) and external (such as travel bans for foreign citizens). The IBCs introduced have been argued to be symbolically important, as a means for member states to give a sense of "structure and order" (Thym & Bornemann, 2020, p. 1144), but they were also in many ways more restrictive than before, e.g., for the first time categorically refusing entry to EU citizens (Heinikoski, 2020) and, in several cases, restricting the number of border crossings open for passage. Thus, controls during Covid-19 have been criticised for having far-reaching, unequal, and asymmetrical impacts on citizens in the EU (Evrard et al., 2018;Wolff et al., 2020).
Despite these re-bordering crises, it has also been argued that Schengen has been "impressively resilient" as a system of border governance (Guild, 2021). Explanations for Schengen's resilience have included member state solidarity and commitment (Votoupalová, 2019), the strength of Schengen as a symbol (Guild, 2021), system adaptability (Mantu, 2021), cross-institutional willingness to preserve Schengen (Ceccorulli, 2019), and intergovernmental efforts (Thym & Bornemann, 2020). Several authors have argued that the crisis of Schengen should be understood not as an anomaly or temporary issue, but rather as part of the structure of Schengen, where instabilities or incompleteness in the agreements of European borders lead to cycles of crisis, re-negotiation, and renewed consolidation of the Schengen regime (Scipioni, 2017;Zaiotti, 2013). It has also been noted that some degree of IBC has always remained in place even since freedom of movement was introduced (Casella Colombeau, 2020). Since border control falls within the policy areas that are particularly sensitive to sovereignty demands, it has been argued that the possibility to use IBCs is an inherent strength of the Schengen agreement as this allows for flexibility in an area where integration would otherwise be difficult (De Somer et al., 2020) and that the member states themselves see the internal controls as a measure taken within the room for discretion allowed by the SBC (Votoupalová, 2018(Votoupalová, , 2019. While the extent of controls was expanded in duration and scope (Gülzau, 2021) at the discursive level, increased focus on national security was not accompanied by less commitment to European integration among member states that introduced controls (Pettersson Fürst, 2023). This is in line with previous literature on differentiation, which has argued that it is a "normal feature" of European integration (Leruth & Lord, 2015) and has pointed to sovereignty concerns as an important driver of differentiation when EU legislation moves into core state powers (Winzen, 2016).
In sum, previous research has shown that the use of IBCs is part of the legal framework of Schengen, which is available for the member states as a policy tool in times of crisis. At the same time, extensive use of controls, either by several states in an uncoordinated manner or by a few states over a long period of time, challenges the principle of free movement and causes unpredictability and inequality among the citizens to whom this right applies. Furthermore, we have seen that periods of crisis and reconfiguration can be understood as part of the system of European integration and that differentiation has become more common as the EU moves into more sovereignty-sensitive policy areas such as border control. Drawing on these findings, the present article aims to contribute by synthesising and moving beyond previous analyses, which have often focused on how the overall use of IBCs impacts European integration (Börzel & Risse, 2018;Gülzau, 2021;Schimmelfennig, 2021) or on specific cases, such as in the group of member states that continue to prolong them since 2015 (Karamanidou & Kasparek, 2020;Votoupalová, 2018). Although several authors have discussed the consequences and implications of the expanded use of IBCs, there have only recently been efforts made to conceptualise the differences in how member states use this legal tool in times of crisis. Guild (2021) discusses differentiation as three regimes, based on whether controls are due to migration, terrorism, or Covid-19. She argues that these three categories have different implications and have resulted in different reactions from the EU, where the former two have primarily triggered cooperation on external border controls and the latter resulted in more "robust" responses and coordination focused on maintaining free movement at the internal borders (Guild, 2021, p. 403).
While Guild's article is a major first step in systematically illustrating the different grounds for reintroduction as three different regimes, her main focus is the different responses from the EU institutions that these different causes for controls have triggered. The purpose of the present article is somewhat different as it aims primarily to highlight and illustrate how member states systematically differ in how they implement controls along two theoretically generalisable dimensions of control. By developing a typology of nested border regimes in the Schengen Area, the article enables the identification of groups of member states that have similar practices, illustrated by the two recent re-bordering crises, but it is also intended to be generally applicable to situations of internal re-bordering that might occur in the future. This provides an analytical framework that can be used to further examine drivers of differentiation (e.g., by identifying good cases for cross-comparison), which in turn is necessary for our understanding of the complexities of the challenges that the Schengen Area is facing.

Schengen as a Border Regime
According to Krasner's (1982, p. 186) definition, international regimes are composed of "sets of principles, norms, rules, and decision-making procedures around which actors' expectations converge in a given issue area." Within the Schengen regime, the overarching principle is the freedom of movement of persons, which is regulated by the norms outlined in the Schengen agreement. These norms define rights and obligations, including the right of free movement for Schengen citizens between member states and the obligation of each state to control its external borders. The rules and procedures laid out in the SBC govern expected behaviour, such as when and how IBCs can be introduced. Regimes emerge and are sustained by repeated patterns of behaviour or practices that are in line with its principles and rules (Krasner, 1982). The development of Schengen as the system of governing European borders established what Zaiotti (2011, p. 14) calls a post-national regime, a "Schengen culture of border control," replacing the previous (Westphalian) system of national border control that became accepted as the norm of border governance in the EU. As Schengen replaces each member state's previous national regimes of border control with a shared regime so that each member state now has both "internal borders" towards other members and "external borders" located in other member states, they must trust each other to uphold the shared border regime through their border control practices: to keep freedom of movement at the internal borders and to control who enters through the external borders (Zaiotti, 2011). Fundamentally, the crisis of Schengen is about the practices of member states (IBCs) being in conflict with the freedom of movement principle. Understanding different uses of internal controls must therefore include an assessment of whether it is the principle of free movement for citizens or if it is the capacity of other member states to control external borders that is being challenged. Previous research has often focused on the external dimension of the Schengen regime, highlighting the complementarity of the Schengen and Dublin systems, as Schengen "marks the birth of the European External Border as an institution" (Kasparek, 2016, p. 61), which has shifted border control increasingly outwards, externalising it from European territory (e.g., Pacciardi & Berndtsson, 2022). This external border regime has received critique for being "at odds with the humanist values that the EU is supposed to uphold" (van Houtum & Bueno Lacy, 2020, p. 706), not least by contributing to the securitisation of migration (Bigo, 2014;Léonard & Kaunert, 2020).
Importantly, while the Schengen regime is a "paradigmatic case" of differentiation in the EU, as some EU members remain outside, most studies of Schengen as a border regime focus on Schengen as an integrated unit, as its external border becomes increasingly demarcated. Bruns (2019, p. 510) identifies a "double mechanism" of homogenisation and externalisation as constitutive features of Schengen's external border regime. The internal border regime of Schengen is also often conceptualised as unitary, even in analyses of differentiation and internal re-bordering (Schimmelfennig, 2021). The problem of treating internal re-bordering as a unitary problem is that it might overlook essential differences in what drives the use of IBCs and thereby treat all use as the same kind of problem. A primary function of any border regime is to determine inclusion and exclusion (e.g., Berg & Ehin, 2006). Thus, what or who to be excluded becomes important for the analysis of internal re-bordering in the Schengen Area. Here, it is argued that two dimensions need to be taken into account in an analysis of IBC usage. These dimensions are derived from the fundamental character of Schengen as a post-national regime that has established a new and shared external border, and they capture the question of whether the use of IBCs fundamentally targets problems that are internal to the Schengen Area or related to the shared external border.
Finally, although the use of IBCs is identified as a challenge to the Schengen regime, it must be recognised that its legal framework allows for the temporary use of IBCs. Thus, we must also recognise that this is not an absolute problem, meaning that there are degrees to how problematic the use of internal controls is. In the literature on migration, the Covid-19 crisis and internal re-bordering, the extent of controls over time, and the number of member states that use them are usually what constitutes the problem. In contrast, the normal use of IBCs before 2015 has not been portrayed as a crisis of the Schengen regime.

A Typology of Differentiated Implementation of the Schengen Border Regime
In the typology presented here (Table 1), differentiated implementation is conceptualised along two dimensions of control: on the one hand, whether the state introduces IBCs to control movement internally towards other Schengen citizens, and on the other hand, whether the state introduces IBCs to control the movement of people who are not citizens of the Schengen Area, a movement that (at least in theory) originates outside the Schengen Area. These two dimensions are important in highlighting that there are qualitative differences between these controls: One is, in principle, about the lack of trust in the external border control capacity of Schengen, whereas the other is, in principle, about controlling internal crossborder movement. Combined, these two dimensions render four different internal regime types: (a) a freedom of movement regime among member states that do not use the discretion they have within the SBC to introduce controls, thus upholding the principle of free movement across internal borders; (b) an external threat regime, where controls are imposed only towards perceived "risky subjects" who are not EU citizens; (c) an internal threat regime, where controls are imposed toward the general population of internal border crossers, and finally; (d) a re-bordering regime, where internal controls are imposed that target both general internal populations and border crossers aimed at non-EU citizens.
Having identified two dimensions of control that differentiate the use of IBCs based on whether their primary function is to control movement external to or internal to Schengen, the next question to understand border regime types is how extensive IBCs need to be in order to pose a challenge to the freedom of movement of persons. It can be argued that controls are a problem regardless of duration, but previous research has highlighted that controls are particularly problematic when states prolong them beyond the total duration stipulated by the SBC (e.g., De Somer et al., 2020;Montaldo, 2020). The average duration of internal controls before 2015 was only 10 days (Pettersson Fürst, 2023), which is far shorter than the maximum of 30 days that the SBC allowed for. In contrast, since 2015, several member Table 1. Typology of internal border regimes.

Not used Used
Controls primarily aimed at internal border crossers (EU citizens)

Used
Internal Threat Regime Re-Bordering Regime states have controls for six-month periods that have been continuously renewed. If we understand regimes as a combination of norms and rules that are sustained by repeated patterns of behaviour (e.g., Krasner, 1982) and border openness as a function of implementation practices over time (Berg & Ehin, 2006), then the more persistent internal controls are, the more they will challenge the freedom of movement regime, and the more established the internal regime type will become. An analysis of internal border regime types will therefore need to identify and take into account differences in the duration of controls.

Method and Material
To test this typology on the use of IBCs in Schengen, a dataset was compiled using the European Commission's (2023) official list of the use of temporary IBCs from September 2015 to December 2022, and a dataset, constructed from the member state notifications available in the European Council archives. During this period, the Commission lists 321 notifications of introductions (or of prolongations) of IBCs by member states. In the present analysis, 264 member-state notifications (representing 82% of the controls) were analysed. Some notifications analysed are not included as separate introductions by the Commission, and for some member states, notifications from 2021 and 2022 were not available in the official archives. The analysis focuses on differentiation within Schengen and therefore excludes EU members that are not part of Schengen. Croatia joined Schengen in January 2023 and is therefore not part of the analysis here. The two dimensions of control were operationalised as follows: The internal dimension was operationalised as the use of controls that primarily aimed to reduce overall movement across an internal EU border, where member states primarily target EU citizens from other member states, and the external threat dimension was operationalised as use of controls that primarily target movement perceived to be originating from outside the EU. During and following the refugee crisis (from September 13, 2015), states that introduced IBCs perceived the threat that motivated controls as originating from outside the Schengen Area, as "unprecedented flows of refugees," "illegal migrants," or "secondary movements" inside the Schengen Area often due to "shortcomings at the external border." The main target of these controls is external, as it is specifically to control the movement of people who are seen as outsiders to the European community, even though controls are performed at the internal border and therefore, of course, also have an impact on all border crossers, regardless of citizenship. In contrast to Guild (2021), who categorises controls due to migration and terrorism as different types, both migration and terrorism-related controls are placed here in the external control dimension for two reasons. First, the majority of notifications that use terrorism as grounds for IBCs do so in reference to threats that are framed as originating outside the EU, for example, that terrorists may be among groups of migrants who can enter because of shortcomings at the external border, that ISIS/ISIL fighters are returning from Syria, or that terrorists can take advantage of secondary movement in the EU (e.g., France). Second, several member states that continue prolongation of controls for several years shift from migration towards terrorism as grounds for controls over time, often citing secondary movements, irregular migration, and the risk of terrorism, as reasons for controls (e.g., Austria, Denmark, Sweden, and Norway), which indicates that these controls have the same foundational motivation. Thus, all notifications that emphasise migration, secondary movement, the overall security situation outside Schengen borders, or terrorism that is mixed with migration or seen as originating outside the Schengen Area, were coded as belonging to the external dimension.
In contrast to the IBCs due to migration, the controls introduced during the pandemic (first used on March 12, 2020) were not about shortcomings at the external EU border. Instead, they were often aimed at reducing all movement across the internal border. The targets of these controls were more generalised, not aimed at stopping people who were perceived as "external" to the EU, but rather to stop the virus, often by closing the border to all but a few categories (such as essential medical personnel). Many of the notifications from this period also include a reduction of the number of available border crossing points in a way that had not been done before. Controls due to Covid-19 and controls due to other reasons, such as high-level meetings where no specific group was explicitly targeted, were both coded as belonging to the internal dimension.
Coding of who was the target of controls was done based on member state justifications of controls in their notifications, i.e., what or whom they identify as the threat and any specific group of individuals specifically mentioned as targets of controls. All member states were coded for whether or not they used IBCs for each reason, how many times and for how long their total duration of controls lasted during the period in question. As a measure of the regime strength, the difference between the four regime types is operationalised here as the average total extent (in days) that border controls have been in place since the first introduction for that reason, calculated as a percentage of days with controls.

Analysis: Nested Border Regimes in the Schengen Area
First, let us look at how many times member states have used controls since the first wave of internal re-bordering in 2015. As shown in Figure 1, it is immediately apparent that there is a huge variation in the number of times that controls have been introduced, ranging from no usage to more than 40 times, with some countries having introduced controls on a few occasions but most countries having used them several times (on average 12.9 times). This illustrates what previous research has called the "new normal" (Gülzau, 2021), namely that the introduction of controls is not only done by a few states on a few occasions but by most states at some point and on several occasions. However, as Figure 1 also illustrates, there is quite a lot of variation with regard to the reasons member states have introduced controls. In Figure 1, each introduction is only coded as having one main reason. However, in the analysis of member state notifications, it becomes apparent that some countries use more than one ground to justify the introduction of controls. For example, both France and Denmark do refer to the Covid-19 situation when prolonging their controls, even when the primary reason they give has to do with risks related to secondary movements. Table 2 below organises each state based on how the detailed coding of each member state notification identified external and internal dimensions of  Table 2, there is quite a clear differentiation of usage, where four distinct groups of countries can be identified. Some member states (upper left corner) have never used IBCs: neither to control migration nor to prevent internal movement during the pandemic. These member states thus uphold the Freedom of Movement Regime type. The second group of states (upper right corner) are those that introduced temporary IBCs to control migration or so-called secondary movements inside Schengen but that did not introduce controls specifically to prevent the spread of Covid-19. These member states thus use temporary IBCs to perform a border function that is normally associated with external state boundaries and are therefore categorised as the External Threat Regime type.
The third group of countries (lower left corner) are those that have not used IBCs to control external movement but have done so to control internal movement as a response to the Covid-19 spread or high-level meetings, and thus belong to the Internal Threat Regime type. In the short term, the extensive controls due to Covid-19 presented a very serious violation of the principle of freedom of movement. However, many countries had also introduced restrictions on free movement locally or regionally, which indicates that this was not a freedom of movement crisis isolated to the Schengen regime. In the fourth group (lower right corner), we have those member states that have used IBCs that target both external and internal movement. In this group are countries that introduced IBCs because of the refugee crisis in 2015 or the Ukraine war in 2022 and also used controls to reduce the spread of the pandemic. These member states challenge the freedom of movement principle based both on external and internal border functions and thus belong to the Re-Bordering Regime type.
There are clear internal differences among these four types. Overall, six countries (Austria, Denmark, France, Germany, Norway, Sweden) stand out as long-term users that have had controls in place for more than seven years each since first introducing them in 2015, and thus more permanently challenged the principle of a free movement in the Schengen Area (Figure 2). Another group that stands out are those that have never used IBCs during this period of time (Greece, Latvia, Luxembourg, the Netherlands), or only very briefly (Italy, Slovenia). Most countries have used IBCs for a period between 100 and 200 days. If we couple this with the typology based on the border control function (Table 2), we can measure the degree of openness for each of the internal regime types. The average number of total days that each group has used border controls varies from zero to 1,664 days (Table 3). In Table 3, the total average share of days with border controls within each group presents these differences. The freedom of movement regime countries have per definition not used IBCs during this period. In contrast, the two regime types that include the external dimension controls have, on average, been used for a large share of days (note, however, that the in-group differences are quite large in these two types as some member states coded in this group did not start migration controls in 2015 but in 2022 due to the war in Ukraine). This indicates that these two regime types have a clearly developed pattern of using IBCs, which means that these internal border regimes are fairly well established. In contrast, the countries in the internal threat regime had an average share of 17% of days with controls since the pandemic started, indicating that although very disruptive in the short term, this regime  type cannot be considered to have established a strong pattern of behaviour.

Conclusion
This article proposed the concept of differentiated implementation to account for the internal border regime(s) that have developed since the practice of using IBCs expanded following the two main crises of Schengen, the "refugee crisis" in 2015 and the pandemic in 2020. Following previous literature on the Schengen crises and IBCs, it was argued that a more nuanced understanding of internal re-bordering was needed, beyond whether the implementation of IBCs will mean disintegration or not. A typology of nested internal border regimes was developed, which highlighted the two dimensions of border control functions: One is, in principle, about lack of trust in the external border control capacity of Schengen, whereas the other is, in principle, about limiting internal cross-border movement altogether. Or put differently, one is primarily about an external dimension of control while the other is about an internal dimension. The analysis then used the typology to identify the ways in which IBC use is differentiated among the Schengen member states. Importantly, it is clear that not all member states implement IBCs in the same way. This kind of internal differentiation risks causing imbalance and dysfunctionalities within the Schengen system as a whole, not least as it makes travel to some countries more difficult than others and disrupts cross-border region-building, which has been a major outcome of the freedom of movement policies. The difference between IBCs that are primarily used to target "external" border crossers and controls that broadly target all internal border crossers is important to make note of. These are, in essence, policy responses to very different types of problems for the Schengen Area and treating them as a unitary problem might obscure possible solutions. It was also argued that the regime strength of the four types was not the same and that the use of IBCs to target freedom of movement during the pandemic has not been developed to an equally established regime type as the external threat control regime types.
The typology presented in this article identifies different internal border regimes of member states based on two dimensions of control, capturing that IBCs can either be directed towards internal cross-border movement or at movement internal to the Schengen Area that originates outside of it. This has been the case in the two previous internal re-bordering crises, where the controls after the refugee crisis have been about shortcomings at the external border, and during the Covid-19 pandemic, where controls were used to reduce internal movement altogether. However, the intention of outlining the external and internal dimensions underlying the use of IBCs is that these two dimensions are theoretical abstractions that can be generalised to a number of other situations where member states choose to introduce controls at the internal border. The typology presented here could therefore be used as an analytical framework for analysing and comparing current re-bordering to any future re-bordering developments. This typology of differentiated implementation could also be used in comparative studies of country cases of different types to explore potential explanations such as if or why similar countries make different re-bordering choices.