Jeff probably noticed they are just PIX files.
They are all 256 pixels wide and their height define how each color of the palette fades into the depth (be it fog, darkness, acid, etc.).
I'm not sure yet how they get loaded and overwritten in the game but it should be possible to create neat effects with it. For example if you were making a very hellish/gore level you could include a new shadetab fading all colors into red or black through red. Or I could tweak my SinCity mod using a custom shadetab to make the red colors very persistant while everything else would quickly fade into the darkness.
Anyway here are some examples from the game:
The first one is the DRRENDER.TAB (flat, no fading)
then a custom one I found into COAST.TAB
DEPTHCUE.TAB
FOG.TAB
ACIDFOG.TAB
BLUEGIT.TAB
