Fourth official

In association football, an assistant referee (known as a linesman or lineswoman before 1996, expressions which are still in common unofficial use) is an official empowered with assisting the referee in enforcing the Laws of the Game during a match. At most organised levels of football the match officiating crew consists of the referee and two assistant referees, with one assistant referee assigned to each touchline. The assistant referee's duties generally consist of judging when the ball has left the field of play – including which team is entitled to return the ball to play, judging when an offside offence has occurred, and advising the referee when an infringement of the Laws of the Game has occurred out of his or her view. Depending on the local match rules and the discretion of the referee, an assistant referee may also be responsible for various administrative tasks, for example managing substitutions, helping the referee control the players, or replacing the referee if he or she is unable to continue. The assistant referee functions in an advisory role, and all judgements made by an assistant referee may be overruled by the referee.

At higher levels of play the referee is also assisted by a fourth official. The fourth official's duties are usually administrative in nature and vary depending on the match rules and the discretion of the referee. Common duties for a fourth official may include managing technical areas and substitutions, maintaining a backup record of the score, and cautions and sendings off, and displaying information on substitutions and extra time to spectators. Depending on the local match rules, the fourth official may replace the referee or one of the assistant referees if they are unable to continue.

Law 6 of the Laws of the Game specifies that "two assistant referees may be appointed". It also outlines the general duties of the assistant referees; however, their duties in a given game remain subject to the decision of the referee. These duties usually include indicating:

All decisions by the assistant referees are only advisory to the referee, i.e. their decisions are not binding and the referee is allowed to overrule an assistant. An assistant referee may also be called upon by the referee to provide an opinion regarding matters which the referee requires clarification on. Occasionally the assistant referee will assist in player management during free kicks, as well as provide visual assistance during penalty kicks. The assistant referees also usually assist the referee with preparatory and administrative functions.

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Wikipedia