Once you get into the Realms Manager, you can create up to 5 Oasis. You pick exactly what map each Oasis gets. Then, you can activate the Oasis, which "Pushes" it to Nitrado. Then you can log into it. If you activate a different Oasis, it "Pushes" that one to Nitrado, and that becomes the One active.
It LOOKS like the system would support multiple Oasis... but unless Nitrado fast-tracks my extra servers, I can't say for sure. From the look of it though, that key you put in tells the cluster manager what servers are in your cluster.