Yeah, I'll try to break it down -

Intro is something you can start casting with 1 rank. Basic is about 10 ranks before you can start casting it at min mana, and Advanced is nearish to 100. Spells that you need upwards of 200 ranks to cast are Esoteric.

In general, this is just a way to call out the vague difficulty of a spell. The distinctions of 'difficulty type' are mostly rough templates to give you an idea of how hard the spell is.

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"The ninety and nine are with dreams, content but the hope of the world made new, is the hundredth man who is grimly bent on making those dreams come true." -E.A.P.