PlayRequirements

Hearthstone powers can have one or more PlayRequirement assigned to them. They determine the conditions that have to be met in order for the power to be usable.

Concretely, they are only used for battlecries / "main" powers, in order to determine whether the card is playable and what it can be played on. Targeting is a separate step from the power triggering, and as such, once targeting is successful, an action can proceed on its target even if the target is no longer valid.

PlayRequirements are set in the CardDefs.xml file. Fireplace uses them to automatically determine all the targeting requirements on cards which have them.