Let's talk about magic and spells.

Being a fan of the four classical elements, I am very much in favor of keeping Water, Earth, Fire, and Air as the schools of magic. But to avoid any of the schools being inherently superior / inferior to others, each can be assigned its specific role.

What is magic usually used for on the battlefield? Well, virtually anything, but as long as we're fitting stuff into boxes, let's split them into Offensive and Defensive spells. On the other hand, by the nature of the spells, they can be used to strengthen friendly units or weaken enemies. Both of those can be used offensively and defensively, so I think it's fair to say we got ourselves a new axis, Buffs / Debuffs.

In general, an offensive spell lets you deal more damage, and defensive one will let you avoid or revert damage. So an example of an offensive debuff would be something that lowers the defense of an enemy stack, and an offensive buff would be something that increases speed of your units, allowing them to get closer to the ranged enemy units. You can see where I'm going with this.