My friends and I were playing this and it really came down to the wire on this Subdue option. It says "If the Royal Guard Champion has been deployed, he may perform one action."

My question is whether it means "has been" as in he's still on the board and was at one point deployed. Does it mean he has been deployed THIS TURN and may perform an action.

So, can I use this several turns after he has been deployed or only the turn that he deploys? Can it be used more than once (Not at a time, but just once for several turns in a row.)

It means: If the RGC is on the map, he may perform one action right now when the event says to do it. (I.e. when Subdue is performed at the end of round step of the status phase and if the IP chooses that option.)

RGC is unique, so if he's defeated, he cannot be redeployed in the same mission. And figures not on the map cannot be targets for abilities.

Why wouldn't they just say "The Royal Guard Champion may perform 1 action.", why be so specific? Sound like he would only be able to do it the turn he is deployed.

Because if the Royal Guard Champion weren't on the map, the imperial player would be confused.

Also, it's a mission from the core campaign from 2014. It is evident that wordings hadn't yet found their final form - they hav not even now.