There are no referees in ultimate. Part of the game’s official rules, Spirit of the Game™, is based on the belief that respect and honor between competitors make it possible for control of the game to be in the players' hands. Each athlete is obliged to trust every other player on the field to act as an official throughout play. Players are responsible for making their own calls.
When a player believes that there has been an infraction, they must make the call. A player called for an infraction can either agree that they committed a foul/violation and say "No Contest" or they may "Contest" that call if he/she believes the infraction did not occur.
If opponents cannot agree on the outcome of a call (i.e., the call is "contested"), there will be a do over, where the disc is sent back to the thrower who possessed the disc before the supposed infraction, and play will continue from there
Alternatively, any player directly involved in a dispute may request an observer resolution - a ruling on whether there was or was not a foul on the play. Once a player requests a resolution from an Observer, all players must abide by the observers’ decision.
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