"Opting in" enables a Member State that has decided not to participate in measures provided for in the Treaties to change its mind at any time. For instance, a special regime for Denmark was included in the Treaty establishing the European Community with regard to Title IV thereof, concerning visas, asylum, immigration and other policies related to the free movement of persons. Denmark may at any time give up that regime should the Danish people call for this.
Opting out is an exemption granted to a country that does not wish to join the other Member States in a particular area of Community cooperation as a way of avoiding a general stalemate. The United Kingdom, for instance, asked to be allowed not to take part in the third stage of Economic and Monetary Union (EMU) leading to adoption of the euro, and similar clauses were agreed with Denmark as regards EMU, defence and European citizenship.