What windows are "popup windows"?

[John Found]

In XFCE, compositing window manager, there is an option "Show shadows under popup windows". 
But what windows are considered "popup" in XFCE compositor?

I have windows with override_redirect = 1, that are not decorated by the WM (as intended) but they 
never get shadows under. In addition I tried to set property _NET_WM_WINDOW_TYPE_POPUP_MENU, but
with null result.

Regards
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John Found <johnfound at asm32.info>