Thanks for the answer Torley, it has answered my question.
It is confusing to have these two systems operating, especially when, on the surface, they appear to be the same thing, but at least i understand it now.I can't quite see why it would be necessary to have the two categories when one would suffice and be easier to understand.
Surely there is no reason to be 'age verified' if one is already accepted as 'adult' or vice versa. I guess the only benefit is that mainland owners of less than a whole sim can flag their land for age verification but not for adult but I am wondering how land flagged this way on the mainland is getting by the adult content rules.
Thanks again for the answer.