What are the differences between a hang that prevents Windows from ending a login session and a hang that does not?

Question

Inevitably, I run into programs that will have a user interface that hangs. However, I can divide those programs into two types: There are the hangs that prevent a Windows session from ending, and there are the hangs that do not prevent a Windows session from ending. How can that difference be used to help diagnose the root cause of the hang?