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The UEFA governing body has introduced a new rule that allows for a fourth substitute in a match as against the conventional 3-man substitute. The standard football rule is that teams or clubs in any competitive matches is allowed to make only three substitutions but now the they UEFA has made an alteration to the rule.
The new rule allows firstly for a fourth substitution which can only be made in the Knock out stages of the Champions league, Europa League and the UEFA super cup (UEFA competitions); secondly the fourth substitution can only come in if a match goes on into an extra time. The rule is also Reported going to apply for women's competitions.
Other UEFA competitions such as UEFA Youth league and Youth European international football will also be applying this rule. This rule was also used in the World cup (2018 Russia)