Overlays are 2D objects that cover all 3D content
(also can be a background of 3D scene, see Background
style). Overlay can
be Text, Image, etc.
A overlay object is a Command Target, can respond to messages and issue commands. But overlay has Transparent style by default, that is don't accept any message. Uncheck this style to resume it's command function as needed.
By default, overlays drawing is after post processing; but if the console variable r_overlay_post is 0, then drawing is before post processing. Also there is a console variable r_overlay for disable/enable all overlay drawing.
As a special usage, you can let a overlay be drawn before 3D content, that is overlay become a background of 3D scene. To achieve this purpose, you just need check overlay's Background style in it's Properties Panel.
Overlay's Z Order