A "Magic-User" class category?
Where have I heard that before? Oh yeah! ARCANE POWER SOURCE!
Personally... I have to wonder though what the point of having this "class category" is. What is gained by it? We can group classes together in all manner of different categories, but unless there's an overriding mechanic to them... there's really no point. We already know that some classes use magic and some classes do not. We already know some classes are granted their magical power from someone else (clerics, warlocks, possibly druids), while some classes get their power from their own self and efforts (wizards, sorcerers, bards, artificers). Do we really need them categorized as such? Especially if that category is nothing more than just a fluffy top-down meta-view of where a class's magic comes from (with no applicable mechanical connection?)
That's just grouping classes together for no necessary reason.