Substitutes are the players who start the match on the bench, rather than on the pitch.
There are two types of substitutes in rugby union - temporary and permanent replacements.
The temporary substitutes come on for any player who has been sent to the blood-bin.
But they must return to the bench once the injured player has returned to the pitch.
But if the injured player has not returned to the field within 15 minutes, they become permanent.
Permanent replacements are substitutes who come on as a direct replacement for a player on the pitch.
The substituted player is not allowed to return to the pitch.