What does the PFD_MAIN_PLANE mean?

I’m using Delphi 5.0 and OpenGL with it.
Can you tell me what does the “iLayerType” field of the PixelFormatDescriptor stands for? One of the valid choose of “iLayerType” is “PFD_MAIN_PLANE”, can you tell me what does it mean?
Thank you!

MSDN Quote :

Ignored. Earlier implementations of OpenGL used this member, but it is no longer used.

EDIT : ok, looks like MS changed its docs… The online MSDN says that iLayerType can be MAIN, OVERLAY or UNDERLAY. Doesn’t say it is ignored anymore… Anyway… MAIN_PLANE means that the OpenGL window will share the usual graphics layer (== Windows frame buffer). Overlays and underlays can be used for special compositing effects. Never seen any sample code using them though.

[This message has been edited by kehziah (edited 07-10-2003).]

Thank you!