Checklist cards are used by double-faced cards and meld cards to provide the standard missing card back these cards are naturally lacking.
Checklist cards have a list of all double-faced cards (common-uncommon/rare-mythic in Shadows over Innistrad) in the respective set on one side and the typical Magic card back on the other side.
In Ixalan and Rivals of Ixalan boosters, double-faced cards (almost) always appear with a checklist card.
From the glossary of the Comprehensive Rules (January 24, 2020—Theros Beyond Death)
- Checklist Card
- A game supplement with a Magic card back that can be used to represent a double-faced card or meld card. See rule 713, “Checklist Cards.”
From the Comprehensive Rules (January 24, 2020—Theros Beyond Death)
- 713. Checklist Cards
- 713.1. A checklist card is a game supplement that can be used to represent a double-faced card or meld card.
- 713.2. A checklist card has a normal Magic card back. The face of a checklist card is divided into sections. Each section lists the name and mana cost of each double-faced card or meld card it could represent and includes a fill-in circle. Before a checklist card can be used, exactly one of the fill-in circles must be marked to denote which card the checklist card represents.
- 713.3. If a checklist card is used in a deck, the card it represents is set aside prior to the beginning of the game (see rule 103.1a) and must remain available throughout the game. A checklist card can’t be included in a deck unless it is representing a double-faced card or a meld card.
- 713.4. For all game purposes, the checklist card is considered to be the card it’s representing.
- 713.5. If the checklist card is face up in a public zone, it should be set aside and the double-faced card or meld card that it represents should be used instead.
- It's important that the cards in your deck be indistinguishable from one another. To accomplish this with double-faced cards, you can use either sleeves, or the checklist cards that are included in some booster packs of sets featured double faced cards.
- You must have a actual copy of double-faced card that the checklist card is representing with you for each checklist card used. For example, if you use four checklist cards to represent Jace, Vryn's Prodigy , you must have four actual copies of Jace, Vryn's Prodigy too.
- The double-faced card should be kept apart from the rest of your deck. In tournaments, the double-faced card should also be kept separate from your sideboard.
- A checklist card can't be included in a deck except when it's being used to represent a double-faced card. If you opt to use checklist card to represent a certain double-faced card, all copies of that double-faced card in the deck must use checklist card instead of actual copy too. (You may use checklist card to represent a double-faced card, but not using them for another double-faced card that has a different name)
- You must mark exactly one fill-in circle on the checklist card to indicate which double-faced card it represents.
- You can still use card sleeves, even if you also choose to use checklist cards.
- During the game, a checklist card is considered to be the double-faced card it represents. For example, say you have a checklist card in your hand representing Tormented Pariah and an opponent casts Despise. The checklist card is a creature card, so your opponent may choose the checklist card and you would discard it.
- As soon as a checklist card enters a public zone (stack, battlefield, graveyard, or exile unless it's exiled face down/manifested), use the double-faced card and set the checklist card aside. If the double-faced card is put into a hidden zone (hand or library), use the checklist card again.
- If a double-faced card is exiled face down or being manifested, keep its identity hidden by using the face-down checklist card.