It isn't a specific disorder. That's why they say 'not otherwise specified'. It means that your mood is outside what the psychiatric profession regards as normal (whatever that is) but it doesn't meet criteria for other mood disorders. When I was first in hospital, I was diagnosed with that. When I was hospitalised a different time, they decided to change the diagnosis to bipolar, but I really don't think there's a lot of science behind which category they decide to lump you in. It's basically a way to put a diagnosis there when you don't fit one of their diagnostic categories.