A texture Challenge

I am a moderator for the Bryce forum and gallery at 3D Commune. We’ve had a few impromptu challenges recently.  Our dear Pinhead offered up the most challenging one yet. Make a texture using all 4 channels in the texture editor. That’s easy enough to do even if you make a bad one. However then we are supposed to figure out the use of the D channel. I’m completely lost. I know that you can use A, B, & C together in any of the categories; diffusion, ambiance, etc, but..if you put channel D there it removes the others, you cannot combine it. So what does that mean and what is the use of it? I have no clue! Here’s a dragon that I textured. All parts have used 4 channels. I’m still working on him.