Rule 420.5
If an effect refers to a card being played and doesn’t do so by means of checking whether cards were played, it refers to the card being added to the chain and queued for Finalization, that is, the Play Game Action. Example: Astral Heron is a unit that reads “When you play your first card each turn, if I'm at a battlefield, the next card you play this turn costs [2][A][A] less.” This effect applies a discount to the next card the play game action, discretionary or limited, is performed on.