§ 128.6 Core Rules PDF (officieel)

A player cannot be compelled to perform an action on cards whose privacy is secret or private if that action specifies a type or quality of card. Example: A card effect reads, “Play a unit from your hand, ignoring its costs.” Although the effect does not say “may”, the player can choose not to perform the action even if they have a unit in their hand, because the effect specifies a card type.