When am I allowed to drag a space/app between displays? When I go into "Mission Control" mode, some full-screen apps and other spaces can be dragged between my two displays, and some cannot -- they just stop at the edge.

I haven't figured out the pattern yet.  Some spaces can be dragged between displays, yet other spaces that seem very similar (same kinds of windows in them) cannot, even in the same Mission Control session.

What's the rule?  Why can't I drag things between spaces, sometimes?
After playing around with it some more, I have discovered the pattern.  (It was tricky to figure out because it changes!)

You can't drag the current desktop (of any display) off of that display.