21 Nobility and Royalty

As SerBear mentionned from Wikipedia, Nobility and Royalty are different. Should we make that difference here?

For now that's not clear as Category:Nobles contain things like :

Also many characters like Aielef du Marquin have the Nobles category.

Should we create a Category:Royalty for royals? Emperors and Empresses should probably be only part of Category:Rulers I suppose.