How do substitutions and the interchange area work?

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Players may interchange at any time, and there is no limit to how many times a player can interchange. Off-field players must stay in their team's interchange area for the whole match. It is a marked rectangle on each side of the field, usually 20 metres long and no more than five metres wide, extending ten metres either side of the halfway line and at least one metre from the sideline. An interchange can only happen after the player leaving the field has entered the interchange area. Players leaving or entering must not hinder or obstruct play. Players entering must take an onside position before becoming involved in play. The penalty for either is awarded to the non-offending team at the point of the infringement. After an intercept or a line break, no interchange is allowed until the next touch or the ball is dead. If a player enters the field in that time and prevents a try, a penalty try is awarded and the player goes to the sin bin. If they do not impede a try, the player still goes to the sin bin. After a try, players may interchange at will without waiting for the leaving player to enter the interchange area, but must do so before the tap is taken to restart play. A team has a maximum of eight substitute players. Source: FIT Playing Rules 5th Edition, definitions and rule 17.