Of the three referees that call a soccer game, only two of them holds flags. These referees run along the sidelines and are called assistant referees. They use their flags to call corner kicks, goal kicks, offsides, throw-ins, and fouls. When calling corner kicks, the referee will point their flag down at a forty-five-degree angle toward the corner flag. On a goal kick, the referee will point their flag toward the 6-yard box where the kick should be taken. If the referee sees a player offside, they must wait for the play to finish before calling the offside. They call this by raising their flag at a ninety-degree angle in front of their body while standing parallel from where the offsides occurred. For throw-ins, the referee must point their flag along the sideline, marking which team gets the ball by pointing toward whichever direction that team is attacking. Assistant referees can also call fouls that the center referee might not have seen.
Flags can also be electronic with a button that referees can press. When they press this button, the referee at the center of the field gets a buzz on their wrist and knows the sideline ref has made a call. This way, the center ref can always be in communication with the assistant referees so that they don't miss a call.