Overlays
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.
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.