Rule · Chapter 1
cr:1.8.6
If the parameters of an effect are undetermined at the time the effect is generated, the player instructed by the effect determines the parameters of the effect. If no legal parameters exist, then any part of the effect related to those parameters fails.
Example: Remembrance has the text “Shuffle up to 3 action cards from your graveyard into your deck.” This effect does not require targets when Remembrance is played on the stack, so when the effect is generated upon resolution, up to three action cards must be determined by the controlling player.