Subrule · Chapter 5
cr:5.1.2c
If the card is a split-card, the player declares which side the split-card will be played with.[9.2.3] If the split-card has meld[8.3.38] the player may declare that the card is played with both sides. The declared side(s) determines the card’s properties when put onto the stack and any subsequent effects applied to it.
Example: A player may play Null||Shock by declaring one of its sides (Null or Shock), or by declaring both sides using meld. Doing so determines the name, types, and abilities of the card on the stack until it ceases to exist.