I do think Bipolar is more of a spectrum thing; but on a grid with one line being manic and one line being depressive. The highest of your mania and the lowest of your depression would draw a line and show you where you fall. I really don't think it's an either/or situation because as we discover more about it, we discover new types of the disorder. While I have never heard of sub-threshold bipolar disorder, it doesn't surprise me that such a thing exists.