The rule is Corvina's and it is written on the last page of her own notes in the same small hand.

CorvinaOne at a time. No choosing by convenience. Whoever is in the room is the one who was asked.

MioneAnd who asks.

CorvinaShe asks. It is the only part of this that is hers.

Malfoid does not know the rule exists until she breaks it in a way that will take two hundred words to explain.