“ Any Member State which wishes to participate in enhanced cooperation in progress in the framework of the common foreign and security policy shall notify its intention to the Council, the High Representative of the Union for Foreign Affairs and Security Policy and the Commission. ”

“ Expenditure resulting from implementation of enhanced cooperation, other than administrative costs entailed for the institutions, shall be borne by the participating Member States, unless all members of the Council, acting unanimously after consulting the European Parliament, decide otherwise. ”

“ Article 328 (ex Articles 27a to 27e, 40 to 40b and 43 to 45 TEU and ex Articles 11 and 11a TEC)
1. When enhanced cooperation is being established, it shall be open to all Member States, subject to compliance with any conditions of participation laid down by the authorising decision. It shall also be open to them at any other time, subject to compliance with the acts already adopted within that framework, in addition to those conditions.
The Commission and the Member States participating in enhanced cooperation shall ensure that they promote participation by as many Member States as possible. ”