386.2. If such a Triggered Ability says "you may," or "they may," as the first part of its effect, or such a Replacement Effect says a player "may" replace the qualifying event, the controller of the ability has the choice of whether or not to perform or apply it.
386.2.a. For Replacement Effects, when an event the Replacement Effect could apply to occurs, the player who controls the Replacement Effect may choose not to apply it to the event. For Triggered Abilities, during Finalization of the Triggered Ability, the player who controls the Triggered Ability may choose not to perform it.
386.2.b. If they do not apply the Replacement Effect, it has not been applied this turn. If they do not perform the Triggered Ability, it is removed from the chain. Example: Zilean, Time mage reads "Once each turn, if you would play a token unit while I'm at a battlefield, you may play that token and an additional copy of it instead." When his controller plays a token, they can choose not to apply the replacement effect to that event. If they do, they can choose to apply it to a later event of a token being played. Example: A player controls a unit that reads in part "Once each turn, when an enemy unit dies, you may banish it." When an enemy Recruit token dies, the triggered ability goes on the chain. If they choose not to perform the ability on finalization, it is removed from the chain. When a Stalwart Poro dies later in the turn, they can choose to perform it then.