The Domain attribute also controls how a Frame behaves when it is used (by AST_FINDFRAME) as a template to match another (target) Frame. It does this by specifying the Domain that the target Frame should have in order to match the template. If the Domain value in the template Frame is set, then only targets with the same Domain value will be matched. If the template's Domain value is not set, however, then the target's Domain will be ignored.
AST A Library for Handling World Coordinate Systems in Astronomy