(glut) what is overlay?

The glut docs (chapter 5) mention “overlay”.

I see no reference to what it exactly is – not there, not in the Red or Blue books, nor in the SuperBible.

What is overlay?

In general, an overlay is a separate buffer that appears on top of the normal color buffer. It is kept separate so that it may be erased without damaging the regular color buffer.

My understanding is that overlay planes are often somewhat limited in their capabilities (256 color, no z-buffer and/or no stencil, etc…)

Thanks heppe!