Rev stands for Revision, meaning version. As far as wow emus r concerned this has nothing to do with the official client or server releases (note that there is no blizzard made data in any of the files the emu uses, they are all "handmade"). There are still revs designed for 2.1.0 that work just fine with 2.1.3 clients, they just dont have the updated material (as far as the core is concerned, the db is a different story).
What should only concern you is that your core (server) works fine with your db and people can log into your realms (if possible with the latest client version else you're gonna need an older patch or something custom). If people can't enter your realms anymore or your server starts crashing then you should think of updating to a newer rev. That's all.