Rule 420.3.b.2
Such game effects may specify those choices or effects based on the condition of the board. If they do so, that information is checked on finalization of the resulting Game Object, unless it is specified as being checked earlier. Example: Bone Skewer is a spell that reads in part “Choose a battlefield. An opponent reveals their hand. You may choose a unit from it. They play that unit to that battlefield, ignoring any and all costs.” The chosen battlefield isn’t checked on finalization of the resulting game object but when the spell itself is resolving. Example: A spell reads in part “Play a unit from your trash to an enemy occupied battlefield, ignoring its Energy cost.” On finalization, they can choose any enemy occupied battlefield to play the unit to. If no such battlefield exists as the unit is being finalized, it will fail to finalize. Example: Rek’Sai, Swarm Queen is a unit that reads “When I attack, you may reveal the top 2 cards of your Main Deck. You may banish one, then play it. If it is a unit, you may play it here. Recycle the rest.” The attack trigger adds a permission to the normal valid locations for the played unit, that being “here.” The “here” in Rek’Sai’s ability is a referent, whose information is checked when the ability resolves. As such, it will not check on finalization of the unit being played.