Rule 205

An instruction that requires a player to pay resources or spend counters or XP that does not also have a linked Effect, is not a Cost. Example: A card effect reads, “When I attack, you may pay [4][C]. If you do, kill a unit here.” Paying [4][C] in this way is not a cost of the ability, base or otherwise, but a game action being performed by a player. The later instruction checks whether the game action was performed, not whether a cost was paid—if paying [4][C] was replaced for any reason, the later linked instruction will not execute.