Now I just gotta find out where the actual description is :D
Up to four optional, nondisplayable auxiliary color buffers can also be supported. OpenGL doesn't specify any particular uses for these buffers, so you can define and use them however you please. For example, you might use them for saving an image that you use repeatedly; rather than redrawing the image, you can just copy it into the usual color buffers. See the description of glCopyPixels() in "Reading, Writing, and Copying Pixel Data" for more information about how to do this.
You can read the contents of the color, depth, and stencil buffers with the glReadPixels() command. Likewise, glDrawPixels() and glCopyPixels() are available for sending images to and BLTing images around in the OpenGL buffers.