There's the principles used. Similarity vs. contagion vs. identification vs. -- quite a few others.
There's the intention -- protection, order, wilderness, etc. -- which may affect what sort of lore you need to figure out how to apply the principles.
There's the technique -- transformation or alteration or what have you.
And then there's typical use. Household spells. Soldier spells. The first three I suspect I'm going to have to systemize so that the young scholars can study them, but this one I can dismiss with a character's observation that by this classification, you have to learn the spells by rote, they don't really have an organizing principle.
