What is the exact purpose of the _sk texture?

[DogtoothCG]

I'm working on some stuff, and I would like to put these to good use, however, they don't really seem to do anything. Based on the ruddy red of the map, and the red that shows in the rimlighting for characters under certain conditions, I assumed (incorrectly, I once again assume) that they acted as some sort of rudimentary light scattering map. However, testing this theory, by replacing the headhuman_sk texture file with both a full white, and a full black texture, with no change in appearance, I am a bit at loss as to what they in fact do.

 

Anyone with knowledge of this, please enlighten me, as a browse through the search results yielded me nothing but guess work.

 

Cheers,

Payton