cr:1.8.6b

If the parameters of an effect include determining an object, the player can only determine public objects in the arena or on the stack as parameters for the effect, unless stated otherwise.[3.0.3] If the parameters of effect include determining a object in a public zone, the player can only determine public objects in that zone, unless stated otherwise.

Example: Oasis Respite has the text “Prevent the next 4 damage that would be dealt to target hero this turn by a source of your choice,” which has a parameter that can be any card or macro in the arena or the stack, as long as it is public (face-up).

Example: Gravekeeping has the text “When this attacks a hero, you may banish a card from their graveyard.” which has a parameter that can be any card in the hero’s graveyard zone, as long the card is public (face-up).

Example: Boneyard Marauder has the text “As an additional cost to play this, banish 3 random cards from your graveyard.” which has a parameter that requires the player to determine 3 random public (face-up) cards in their graveyard to be banished.