In usual practice, the fourth official assists the referee in the following ways:
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The primary duty of the fourth official is to replace the referee in the event that he (or she) is unable to finish the match. However, the actual duties of the fourth official are much more detailed in practice. Details of the fourth official's duties follow, but the important thing to remember is that he or she is just as important a part of the officiating crew as the referee, the assistant referees, the reserve assistant referees, and in the case of the FIFA-authorized UEFA experiment, the additional assistant referees (located near the goals behind the end line).
Prior to the match, the fourth official participates in the pre-game conference and assists with inspection of the field, game balls, and player equipment.
During the match, the fourth official manages the substitution procedure, maintains order in the teams' technical areas, acts as a liaison between the officiating team and the competition authority, venue staff, security, press, and other personnel who might have business to discuss with the officials during the match, and maintains a backup record of goals scored and sanctions issued.
In the final minutes of each half of the match, the fourth official is responsible for informing the players and everyone else of the number of minutes of stoppage time to be added by the referee for time-wasting, injuries, time-consuming substitutions, and other causes authorized by the Laws.
After the match, the fourth official participates in the post-game conference, compares game notes and facts with the other officials, and cosigns the official match report that is to be submitted to the league or competition authority.
It is important to debunk a common myth, that fourth officials are somehow "lesser" officials than the referee or the assistant referees (or in the case of the UEFA experiment authorized by FIFA, the additional assistant referees). In international play, the referee will be a Grade 1 FIFA Referee (the highest grade), and the assistant referees must be at least Grade 2 FIFA Assistant Referees (ARs). The fourth official must be a Grade 1 FIFA Referee, which is not required for the ARs, because he or she must be qualified to take control of the match if the Referee is unable to complete the match.
The 3rd official is the individual outside the field that holds the electronic board for substitutions and added time.