In a nutshell:
- A characteristic-defining ability (CDA) is an ability that sets information that would otherwise be found elsewhere on the card. Characteristic-defining abilities are special in two ways:
- Characteristic-defining abilities are always on. No matter what zone the object is in, even if it’s outside the game, the CDA applies.
- Characteristic-defining abilities are the first to be applied in each layer when evaluating continuous effects, regardless of dependencies or timestamps.
- An ability is a CDA if it fits these criteria:
- It defines an object’s colors, subtypes, power, or toughness. Examples: devoid, changeling, Tarmogoyf. Non-examples: Pact of Negation (its oracle text uses a color indicator, which doesn’t count as a CDA), Copperhoof Vorrac (doesn’t set the p/t)
- It is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect. Examples: Elephant Resurgence, Clone or Volrath’s Shapeshifter copying a creature with a CDA
- It does not directly affect the characteristics of any other objects. Non-examples: Thran Lens, Urborg, Tomb of Yawgmoth
- It is not an ability that an object grants to itself. Non-example: Myth Realized. The p/t setting ability it gives itself when it animates otherwise would be a CDA, but is not for this reason.
- It does not set the values of such characteristics only if certain conditions are met. Example: Chameleon Spirit. Its ability is always trying to set its power and toughness, even though it will only have a “chosen color” while it’s on the battlefield. In other zones, it won’t have a color to work with, so it will set them to zero. Non-example: Duplicant. Its ability will not even try to set Duplicant’s characteristics unless there’s a creature card exiled with its first ability.
A: No. Lord of Extinction has a CDA that sets its power and toughness. This ability applies even while it’s in the library, so it will have a p/t of 10/10.
A: Yes. Sewer Nemesis also has a CDA that sets its power and toughness. This ability thus applies in the library. While it’s there, though, Sewer Nemesis doesn’t have a chosen player, so the game will have undefined values for the p/t, which it treats as zeros.
A: Yes. Wight’s ability is not a CDA because it doesn’t set the power and toughness to a specific number; it only modifies them. This ability therefore only functions on the battlefield, so in the library wight is a 1/1.
A: Yes. Thassa’s “not a creature” ability is not a CDA because it doesn’t apply all the time, only if a condition is met (that condition is having devotion that’s lower than 5). It also doesn’t set Thassa’s color, subtypes, power, or toughness. It tries to make it lose a card type. Therefore, the ability only functions on the battlefield, not on the stack.
A: Yes. The current Oracle wording of Transguild Courier uses a color indicator to make it all colors. Because of this, Transguild Courier no longer has the ability that did this in the original printing. Color indicators don’t count as abilities, so Muraganda Petroglyphs gives it the bonus.
A: No. Devoid is a CDA, which means it applies everywhere, even outside the game. Glittering Wish will see this card as being colorless, so it’s not a legal card to choose.
A: Clone first becomes a copy of Forerunner of Slaughter in layer 1. Then, in layer 5, both Purelace and the Clone’s acquired devoid ability want to set Clone’s color (Effects from spells that set the characteristics of an object on the stack continue to apply to the permanent that spell becomes). Devoid is a CDA, so it applies before Purelace, which isn’t. Clone starts as blue, then becomes black/red, then colorless before finally ending up white.