I do think there's a spectrum of severity, but I don't think you need to have been hospitalized to have the diagnosis of BP1 or 2. There can be any number of reasons why someone isn't hospitalized and I don't even think severity of symptoms is always the reason.

There are people who are rarely, if not often, stable and I would say this would lie on the spectrum of severe and others whose BP is well controlled and go for long periods of time without having an episode (though, that said, if this person has a very severe episode even after not having had one in a long time, then isn't that severe? I'm getting confused myself here...).

I don't think BP1 is necessarily more severe than 2, since those with 2 spend much of their time depressed.

There's also a minority of pdocs who believe in a spectrum of I 2, and then III and IV as well.