User blog comment:Nameless Wikia contributer/Magic: Specialization or Generalization/@comment-26454558-20161219215211

From what I can tell from the 4th novel, it seems that mages can choose which elemental they want. Shihoru may have chosen Shadow spells at first, but she changes to Lightning spells to become more helpful with the team in the offensive; this proves that mages can use more than one type of elemental (as long as it fit their needs, I guess?), though I'm not sure that Lightning types are subcategories or not (like, something you unlock/can choose after gaining experience..?).

We also get more details on the essence of magic spells in the 6th novel and onwards. When you pay for new spells, it's practically like how Haruhiro learns Thief techniques (tiered-level system), but if have an understanding of magic and adding creativity, you can do so much more. Spells take up lots of energy, so combining with other mages is doable.

I don't remember much, so I'd have to go back to reading the books, but yeah, that the gist of it.