MLS will allow teams to make a fourth substitution during this year's postseason if a match goes to extra time.
The International Football Association Board (IFAB), which approves all changes to the Laws of the Game, has allowed for leagues and tournaments to experiment with using an additional substitute in extra time of games.
Already this year, the experiment has been implemented in the FA Cup, Scottish Cup, FIFA Confederations Cup and FIFA Club World Cup. IFAB is expected to make a final decision on making the experiment permanent in 2018 or 2019.
Under the current MLS playoff format, the rule is most likely to come into play during the single elimination knockout round and MLS Cup final.
The conference semifinals and conference finals are contested over two legs and use the away-goals tiebreaker, making it less likely that the rule will be used during those rounds.