cr:6.6.6a

When a triggered-layer is added to the stack, the player that controls the effect must declare the parameters of all abilities of that layer. If the card has any modal resolution abilities, the player must declare the modes for those abilities.[1.7.5] Then, if any resolution abilities generate effects that require a target, the player must declare all legal targets for those effects .[1.8.5] If no legal targets can be declared, the triggered-layer ceases to exist and is not added to the stack.

Example: Thaw has the text “While Thaw is in your graveyard, at the start of your turn, you may banish it and choose 1: Destroy target Frostbite. Destroy target Ice affliction. Unfreeze target frozen card.” To add Thaw’s triggered-layer to the stack, the controlling player must first select the mode and choose a legal target for that mode, otherwise the triggered-layer ceases to exist and will not be added to the stack.