Essentially, it's a provisional diagnosis that means "It looks like bipolar, acts like bipolar, and feels like bipolar, but we don't know yet if you really have it or what flavor you have". Pdocs like to use this one when your episodes don't fall inside the parameters for either BP 1 or BP 2---like if your hypomania lasts 3 days rather than the 4 required by the DSM, or if your cycles are super irregular, or if you don't respond to medications.