The operation of this Regulation will involve Member States in various instances, for example through the notification of information requests to the Member States concerned and through the sharing of information between the Commission and Member States, thereby reflecting the principle of sincere cooperation between the Commission and Member States set out in Article 4 (3) TEU (available here). (europeanlawblog.eu)
It does not only concern Article 218 TFEU on treaty - making, but also such provisions as Article 3 (5) and 21 TEU on external objectives of the EU at large, or Article 4 (3) TEU on sincere cooperation between the Union institutions and the Member States. (europeanlawblog.eu)
[3] It is reflected not only in the general provision on sincere cooperation, but also in several specific Treaty provisions, such as Member States» duty to re-negotiate or even denounce prior international agreements. (europeanlawblog.eu)