Some Triggered Abilities will trigger “once each turn.” 1. When the Condition is fulfilled, the player who controls the source of the Triggered Ability may choose to place it on the chain. 2. If they do not, it has not Triggered this turn.
§ 383.3.f Core Rules PDF (officieel)
§ 383 Triggered Abilities are repeatable effects that happen when a Condition is met.
§ 383.3 When a Condition is met, a Triggered Ability behaves like an Activated Ability and is placed on the Chain.