Official checklist cards are used to represent double-faced cards in sets that contain them. Only official checklist cards may be used to represent double-faced cards in a deck.
The use of checklist cards is required if a player has double-faced cards in their deck and is not using completely opaque sleeves.
If a player uses a checklist card to represent a double-faced card in their deck, then all copies of that double-faced card in the deck must be represented by checklist cards, and any copies of that double-faced card in a hidden zone are considered to not exist for purposes of determining deck legality.
Each individual checklist card used must have one (and only one) of the items checked.
A checklist card is only used while the card it represents is in a hidden zone. The card represented by a checklist card is not a playable Magic card until the checklist card has been placed in a public zone. Multiple checklists cannot be used to represent a single copy of the actual card. For each checklist card used, the player must have a copy of the actual card available, though they are not considered sideboard cards and are not presented to their opponent.