Hi @Justonma, RS is actually a “sense” input. It allows Hunter to essentially listen for when your rain sensor is active.
I agree with @DLane, looks like white is your “Common” and Black / Gray are your rain sensor wires so follow his advice and your should be good.
If you wish to verify everything prior to making a switch. I recommend you connect the white cable to your C terminal (where the gray one is connected now), this should allow you to run the zone(s) with Black / Gray wires disconnected.
Cheers,
Gene