You must use SSS Set Name in order to spread SSS across multiple objects. For example, from a face to an eyeball.

SSS Weight

The SSS Weight in this case is comparable to having the Diffuse Weight in an Ai Standard shader that only has a Diffuse component. Reducing the SSS Weight value to zero still shows visible shading because the Specular component is still on.

Radius Multiplier

Shallow Scatter, Mid Scatter, Deep Scatter

Example of Ai Skin shader connections

Simplified skin shading workflow

Instead of using three unique textures to control the three skin layers (Shallow, Mid and Deep Scatter) you can simplify the workflow using just one texture. Instead of tinting the files with a painting program, you could try using one diffuse map and then use the Maya multiplyDivide node in combination with a flat ramp color texture to create three different sss layers with the skin shader. Once you are  happy with the skin tone you can then replace the three inputs with three different maps and then add more unique details to the maps. This way enables you to work faster to balance the skin tone without having to keep switching to an external painting program.


Scatter Radius


SSS and Indirect Rays  

SSS is not evaluated in indirect rays traced by the SSS. Indirect rays fired by the SSS engine from one object that ends up hitting another object will return black. This is in part legacy inherited from the point-cloud based SSS engine, and in part a fundamental limitation of diffusion based BSSRDFs (which their inventor, Henrik wann Jensen, always warned were only exact for infinite, semi-infinite media and not for curved/thin surfaces etc). We have investigated how to fix this limitation of BSSRDFs but so far our efforts have not been successful. A workaround is to connect a Standard material to the diffuse attribute of a Rayswitch shader. 

In the example below, you can see that green light is bounced from the cube on the left onto the sphere. However, no light is bounced from the cube when a Skin shader is assigned to it.