Well, I don't know all of the differences they account for to distinguish the type of depression someone has, as I'm not a doctor.
I do know that the only times I was ever admitted was when I was suicidal and/or homicidal. They didn't ever seem to care about the self harm, at least not enough to admit me. When thoughts turn into a plan it needs to be addressed before it turns into an attempt and some doctors will admit someone when said person describes their plan.
It also really depends on the situation as a whole, sometimes.