I've noticed that the skin gets darker when I go from 75 to 25, but I don't know if that's like a bunch of different skin colors, like 75 is a really black person while 25 is a mixed person, or if those values just adjust something like the lighting.
Comments
It is supposed to change the intensity of Subsurface Scattering in the shader.
The higher the number the more scattering occurs.