For most basic spells they can be used as a spell and an enchantment interchangeably, these are multi-usable spells because they can be used for both purposes. But more complex spells are different, as a spell gets more complex it also gets more specific with how it needs to be used, the T.P.T spell was made to be cast by a pony on a pony. And it falls into the Non-usable category because of that.