12 Nautical Miles (12M) limit - Scottish territorial seas - Territorial Sea Act 1987 / Scotland Act 1998
12 nautical miles (from baselines) is the limit of the Territorial Sea which is that part of the sea adjacent to the coast of a state that is considered to be part of the territory of that state and subject to its sovereignty.
This is modified version from the UKHO Law of the Sea department data, available via https://www.gov.uk/guidance/inspire-portal-and-medin-bathymetry-data-archive-centre