I think sometimes the doctor just uses their own opinion, and sometimes they look for certain characteristics. What I've heard is along the lines of: mild tends to be just bothersome and not always requiring treatment, moderate starts to really interfere with work or school, and severe is where someone has bad trouble with even the most basic functioning, like showering or getting out of bed at all.

The point that they usually say requires hospitalization is if you're suicidal and have a plan. If you really feel you need hospitalization, though, you should be able to go in even if you aren't at that point.