What do you mean by different rules? At one level, of course they differ. A rune affinity is a broad magical ability (raisable like a keyword - 2 points a shot) that allows active use of magic within that rune. A charm is an independent ability tied to a rune for descriptive purposes. A spell is a narrow and specific ability that defines a power within a rune and can be actively used within its narrow and specific parameters.
A feat - well, feats are specific deeds or myths of the gods which - although potentially quite broad - are treated as a narrow and specific ability as long as you act in accordance with that feat. The moment you stop doing the feat, it is over for the rest of that scene.
Does that help?
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