Seems to be this is Rule
13.2
In addition to the number of Fielded Substitutes permitted in a Match under Competition Rule 13.1, each Club may use up to a maximum of two Concussion Substitutes in a Match. In the event that a Concussion Substitute is used, the opposing Club may make an additional substitute at the same time or any time thereafter. In such instances, a Concussion Substitute and, where applicable, an additional substitute shall also be considered as a Fielded Substitute.