Question about haze material

I find that materials like Caelum/DepthComposer_SkyDomeHaze in DepthComposer.material has two empty texture units:

texture_unit Screen
{
filtering none
}

texture_unit Depth
{
filtering none
}

So I just want to know what are the inputs of these two texture units? I checked the Caelum source but I can't find relevant code.
Anybody tell me where is the relevant code?