Arbrax is an Imperial Commander turned as an Rebel intelligence asset. He is an Executive Agent willing to pass along information and carry out small tasks. He is NOT a member of the Rebellion.
Fox was planning to turn Arbrax in as a Rebel Agent and prepared some false documents to show he helped plan the theft of the AT-ST shield generators, but the generators didn't show up at the raid as per his plan, and 63 did not ask for more rebels - perhaps Fox himself was counted as a Rebel to satisfy his “7 Rebels for 7 family members” requirement.
In any case, Arbrax remains in his position and may provide intel for the party in the future.