Laws of the Game
1. Assistant referees
They indicate when:
the whole of the ball leaves the field of play and which team is entitled to a corner kick, goal kick or throw-in
a player in an offside position may be penalised
a substitution is requested
at penalty kicks, the goalkeeper moves off the goal line before the ball is kicked and if the ball crosses the line; if additional assistant referees have been appointed the assistant referee takes a position in line with the penalty mark
The assistant referee’s assistance also includes monitoring the substitution procedure.
The assistant referee may enter the field of play to help control the 9.15cm (0.10 yards) distance.