The opt-ins and opt-outs, which appear at different places in the Amsterdam Treaty where the policy related to movement of persons is concerned, are the result of difficult negotiations concerning the future of the Third Pillar, which was introduced by the Treaty on European Union. More particularly, these arrangements are the result of the discussions on communautarisation and the integration of Schengen co-operation in the framework of the European Union. Communautarisation concerns the process of transferring substantive matters from the Third Pillar area to the First Pillar area. This process implies, as a result, that proper Community procedures become applicable in these new policy fields.,
Cambridge Yearbook of European Legal Studies
Erasmus University Rotterdam

de Zwaan, J. (1998). Opting in and Opting out of Rules Concerning the Free Movement of Persons: Problems and Practical Arrangements. Cambridge Yearbook of European Legal Studies, 1, 107–124. doi:10.1017/S1528887000001105