The Subdermal layer
This is the middle or fatty layer and contributes to the red glow effect. To see its effect set Subdermal Scatter weight up to 1.0, and make sure the radius is set to 25mm.
Reducing the Subdermal radius to 1mm produces sharper shadows as seen in the following image.
At the other extreme, increasing the Subdermal radius to 100mm produces a strong scattering and a very soft appearance.
Setting the Scale Conversion to 0.1 produces scattering that is too big and grainy. The result looks like a red balloon as too much of the Subdermal layer (the lower bloody layer) is showing through.
The Epidermal layer
The Epidermal layer is the top skin layer or outer layer. To see its effect, turn the Subdermal and Back layer off. Setting the material to the following settings produces:
Epidermal scatter weight = 1.0
Epidermal scatter radius = 8.0 mm
Subdermal scatter weight = 0.0
Back scatter weight = 0.0
Like the Subdermal layer and Scale Conversion, decreasing the Epidermal scatter radius reduces the scattering effect and increasing the radius enhances it, giving a softer result.
Epidermal scatter radius = 1mm
Epidermal scatter radius = 50mm
After tweaking the three diffuse layers - Epidermal, Subdermal and Back - the following image was rendered, ready for mapping.
Unscattered diffuse weight = 0.4
Epidermal Scatter weight = 0.5
Epidermal Scatter Radius = 8mm
Subdermal scatter weight = 0.5
Subdermal scatter radius = 35mm
Back Surface scatter weight = 0.5
Back Surface scatter radius = 25mm
Back Surface scatter depth = 25mm