The number on the paddle is the number of the player who is being substituted. This is important due to the volleyball rules on reverse substitutions: if in a set, player B replaces player A, then the only substitution which can be made involving players A and B for the rest of the set is for player A to replace player B again. For example, you cannot do the following substitutions within one set:
- Player B replaces player A
- Player C replaces player B
This is covered by rule 15.6:
15.6 LIMITATION OF SUBSTITUTIONS
A player of the starting line-up may leave the game, but only once in a set, and re-enter, but only once in a set, and only to his/her previous position in the line-up.
A substitute player may enter the game in place of a player of the starting line-up, but only once per set, and he/she can only be substituted by the same starting player.