An assistant referee 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.