/window -df @a -1 -1 200 200
creates a window with a titlebar.
/window -df +n @a -1 -1 200 200
creates a window without a titlebar (+x behaves the same).
Regardless of +t, I believe this second example is a bug, if the first example creates a titlebar, the second line should be creating the exact same titlebar.
If +n (or +x) is meant to be used only with +t, then +n/+x would be ignored in this case, but I don't see why it would create a window with no titlebar, this is not documented and is not convenient.
Also, if +n (or +x) is meant to be used only with +t, I would like to see the help file being clear about it.