Assistant referee

An assistant referee, also known as linesman, is a referee that helps the main referee with areas that they cannot see or control. The assistant referee has the power to do things that the main referee cannot do. For example, in Football (soccer), the assistant referee can call offside, determine if a goal was valid based on the ball position, and determine who is in possession if the ball went out of play. However, assistant referees cannot overrule a referee's decision, such as if an assistant believes there was an offside but the referee believes there was not.
General duties.
As outlined in Law 6 of the Laws of the Game, the officiating team works together to enforce the Laws during a game, though the referee remains final decision maker in all matters. All officials wear uniforms that match that of the referee, though the fourth official will often wear a tracksuit over the kit in colder weather.