As a sort of follow-up to this question, what units does the scattering radius use on a cycles subsurface scattering shader?
There is a BlenderArtists thread on this subject. In it it is said:
About Scale: 1 blender unit = 1 meter. (I use metric system) don't know how works with Imperial, never tested.
Scale is actually multiplied by the Radius then, so if you use Scale = 1.0 and Radius = 0.1, 0.1, 0.1 is equal to Scale = 0.1 Radius = 1.0,1.0,1.0
When using colors it's the same but calcs are not that quick, at least for me.
Let's say we want the monkey to sub-surface scatter a bit of red like my example above. Setting Scale = 0.1 And Radius = .5, .05, .05 (RGB!) will give the same output as Scale = 1.0 and Radius = .05, .005, .005 (as you see I divided by 10 the Radius values to compensate the Scale being 10 time bigger)
The radius appears to be derived from both the scale of the object and the radius values. Applying the scale should change results, and is a good way to test this theory.
Scale seems to increasing the number of rays rather than how deep they penetrate the surface. Radius acts more like depth. Its the radius that matters most but the way it works in combination with scale is weird.