444.1. Paying a resource is the act of removing that resource from your Rune Pool.

444.2. When a player is instructed to Pay a resource, that player may remove that resource from their Rune Pool if it exists there. If they choose not to, the instruction is ignored. Example: When Diana, Lunari's triggered ability resolves, if its controller has [1] Energy in their rune pool, they choose whether or not to pay that Energy as instructed. If they choose not to pay the Energy, they will not perform the additional effects of the triggered ability.

444.3. Pay is a Limited Action.