What is this?
When clicking the name I'm not seeing anything different. It still shows the realms off line. In fact, it's the only option. Now my memory isn't completely clear here, as it's something I've always just clicked on, but for my U.S. account the options there have always been "United States" and "Oceanic." So what's this new tag about? The only time I've seen this has been on the PTR.
I have a few guesses.
First, and quite likely the most plausible explanation, is that it is an artifact of the servers being upgraded. Blizzard takes the servers down, the put them into a special mode, and they update them as needed. That could be all this tag is about.