Signature

Signature can apply to any card type; it is not another word for Spell. Verified examples include Lightning Rush (Spell, Kennen), Tibbers (Unit, Annie) and Spinning Axe (Gear, Draven). Each still follows its own card type.

Kennen’s Legend, Heart of the Tempest, permits Lightning Rush through their matching Kennen tag. Merely sharing Chaos and Order with that spell does not let another champion’s Legend include it. Domain identity, normal copy limits and format legality still apply. Three Signature cards means three across all names, and including them is optional.

The artist’s signature and * on a collectible Overnumber are printing features. They do not give it the gameplay Signature supertype. Read the type line and champion tag.