wl_subsurface vs xdg_popup?

[Shawn W]

I've been looking at the protocol docs on http://wayland.app and something that's stood out to me is wl_subsurface and xdg_popup. If I want a pop up menu, which one should I go for? I would guess xdg_popup, but it seems like some compositors may not support repositioning if they don't support version 3 of the interface, and positioning a popup seems a little complicated. Then I look at wl_subsurface, and unless I'm misunderstanding it, it seems like it could also be used for generating popups, and is required for compositors to support, but it's not clear to me whether the compositor will actually show a wl_subsurface, since it wouldn't show a regular wl_surface. Would someone be able to clarify the difference between these?
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