With the referee's consent, roll off, roll on replacements can be made at any moment throughout the game, even players who have already been substituted. The replacement must be of a different nationality to avoid discrimination. The replacement player should be sent off immediately after they come on, and play must stop while the ref changes sides. If the replacement plays well enough to deserve it, then their team will receive a free kick in the opposition's half. This is called an indirect free kick. There are cases where a player is injured and cannot continue. In this situation, the referee may allow the player to be replaced by a substitute. The substitute must wait until the ball has been kicked off before he or she enters the field of play. In addition to national referees' associations, international referees are required to join one of three organizations that promote fair play and the rules of soccer: FIFA, which controls world football; UMPF (Union Mondiale de Patine a France), which oversees football within the European Union; and NAFF (North American Football Federation), which administers football in the United States and Canada. International referees are expected to know the rules of their sport and follow them when dealing with issues before them.