355.17. If a Game Effect has introduced a mandatory requirement on any of these choices, that mandatory requirement must be followed if possible. A mandatory requirement is one that specifies what choice can be made or how that choice is made, but doesn't specifically disallow other game states. Example: Here to Help is a spell that reads in part "You may play a unit from your hand to a battlefield you control." If its controller decides to play a unit, they must play it to a battlefield they control. 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 "you may play it here," portion of this ability is not a mandatory requirement, because the player isn't required to play the unit to that location.
355.17.a. If there are multiple mandatory requirements that do not conflict, players must make a choice that will satisfy those mandatory requirements. Any individual group of such complementary mandatory requirements that contains as many non-contradictory mandatory requirements as possible forms a set. If there are two or more sets of mandatory requirements that are mutually contradictory, choose one of those sets and ignore the rest. Example: Here to Help is played from facedown at Targon's Peak. Its controller chooses to play a Dune Drake from hand. The other battlefield is Dragon Roost. When finalizing Dune Drake, they choose to pay the [A][A] additional cost from Dragon Roost. There are two competing mandatory requirements here: playing Dune Drake to Targon's Peak, or playing it to Dragon Roost. Dune Drake's controller must choose one of those requirements to satisfy, and ignore the rest. Example: If Here to Help were played from hand, and its controller controlled both battlefields, if they paid the [A][A] additional cost, they would have no choice but to play Dune Drake to Dragon Roost as it would satisfy both mandatory requirements.
355.17.b. If there are one or more restrictions on choices being made during this process, those restrictions must be followed. Failure to do so means the card will fail to finalize. Example: Perched Grimwyrm is a unit that reads "Play me only to a battlefield you conquered this turn." If a player plays Perched Grimwyrm via Here to Help and they haven't conquered a battlefield this turn, Perched Grimwyrm will fail to finalize. Although Here to Help presents a mandatory requirement, Perched Grimwyrm's restriction on being played overrides it. Example: Mageseeker Warden is a unit that reads in part "While I'm at a battlefield, opponents can only play units to their base." If a player attempts to play a Perched Grimwyrm to a battlefield they've conquered this turn while their opponent controls a Mageseeker Warden at a battlefield, the Perched Grimwyrm will fail to finalize.