The domestic laws of a country are laws that can be upheld in its courts.

Scots law is the kind of domestic law that’s enforced in Scotland’s courts.

If someone wasn’t keeping promises they’d made under an Act of the Scottish Parliament, or an Act of the UK Parliament that applies to Scotland, they’d be breaking domestic law, and so could be taken to a Scottish court.

If they weren’t keeping promises made under international law this couldn’t happen, unless those promises had also been written into domestic law.