Can you even choose RM-exclusive as such nowadays?
You can tick 'only on Alamy', if you choose; but I think RM-exclusive as an option has gone.
As far as I'm aware, the only advantage of the old RM-exclusive was that in the unusual case that a buyer wanted to buy rights, Alamy wouldn't have to contact you to check whether it had been licenced already - that would rule out your magazine use.
I can't remember for sure whether buyers could ever see that an image was RM-exclusive, but I don't think so. Also as far as I can see, buyers don't have access to the information that a file is 'available only on Alamy' for the present; whether that will change in the future, I have no idea.
Some inconclusive discussion here:
http://discussion.alamy.com/topic/6910-only-available-on-alamy