@Eagle-Eye:
I’m not sure if it’s used for helicopter AAR, so it’s just a guess. I have no idea about fixed wing either. The area is 40NM long and 10NM wide, so technically, it is possible, but I don’t see why fixed wing would want (or dare, given everything that can go wrong) to practice AAR at such low altitudes.
Just checking the (real) Korea AIP, I noticed there’s also a remark saying “1 hour after sunset to 1 hour before sunrise”. To my knowledge, those are operating hours, but that would make this area seem even weirder. Why have anyone refuelling at low level, at night, at +30NM from nearest land? 😕
It is possible this is an area where to train helicopters to perform night refueling with a close recovery airport if something goes wrong. This is my guess!