Brussels IIa Regulation is the cornerstone of EU judicial cooperation in cross-border matrimonial matters (divorce, separation, marriage annulment) and matters of parental responsibility, including custody and access rights, and international child abduction. The Regulation has applied in all Member States, except Denmark, since 1 March 2005.

The Regulation applies only in cross-border cases within the EU.  It sets out rules determining which country's courts are responsible for dealing with matrimonial matters, parental responsibility and child abduction in disputes involving more than one country. It ensures that judgments issued in one EU country are recognised and enforced in another EU country.

The Regulation focuses on settling procedural aspects of cross-border cases only.

Member States retain the sole responsibility of any substantive rules, for example, on how to exercise access rights or on which criteria apply when attributing custody rights to a parent.

More on that: https://europa.eu/youreurope/citizens/family/children/parental-responsibility/index_en.htm