I've been diagnosed with Bipolar II Disorder. From my research and understanding in talking to my pdoc and t, it does mainly have to do with the extremity of your mania episodes. Those of us with Bipolar II tend not to experience the full blown effects of mania, therefore they call it hypomanic. Also, from what I've read, Bipolar II patients tend to spend more time in depressive episodes, as compared to those with Bipolar I.