We've learned that volleyball is a 6-on-6 sport game, meaning there are six (6) players on the court for each team at all times. Since volleyball games aren't timed, sets can run long and players can get tired, so it makes sense that the same six (6) don't play the whole game. They are substituted out.
Substitution is the act of replacing one or more players of a team on the court, with players from the benches, located behind the sidelines. These players have to be on the team roster and noted on the score sheet.
Coaches must request a substitution, which is approved by the 2nd referee. Substitutions must take place in the substitution zone. Each team is allowed six (6) substitutions per set.
You may see players holding paddles with numbers during substitution, these are used by incoming players to tell players on the court who they are replacing.
Players are allowed to leave and re-enter the game once during each set. When a player comes back into the game, they have to return to their original position.
If this rule is not followed, it is considered an illegal substitution.
Exceptions are made for when a player is injured. In this case, the player should be substituted legally. If not, any player, except the libero, can fill the spot of the injured player.
The number of substitutions per set varies per level of play (Youth 18, College 12, FIVB 6)
Replacing the Libero player is not considered a substitution, so it is not counted in the six (6) per game and has some different rules: