Posted this question on my Facebook which Zap Anderson kindly answered for me. ( Thanks also to Jenni O’Connor for the help )
Question: I was wondering if you shed some light on the falloff parameter of the MISSS_Fast_skin shader in maya 2012, I couldnt find any documentation on this parameter, and wondering if its scene scale / rig scale dependent?
Answer: No, falloff isn’t scale dependent. It is really just a “shaping” of the filter kernel. Imagine the blurring-filter for light has weight 1 at the center and 0 at the scattering radius and if it was linear between the two it would basically be a triangle filter?
Except that triangle is raised to the power of “falloff” allowing you to play with the “shape” of it.