The Sub-Surface Lightmap Write shader is the lightmapping shader that is used to create lightmaps for fast subsurface scattering effects. This shader creates a lightmap and stores the front and back surfaces, their depth, and irradiant light intensities in one or more specially formatted lightmaps.
Choose User Tools Create SSS Lightmap in the render tree to create an Image Clip with a Writable Image Source, where you can define the output options for the lightmap image.
The Sub-Surface Lightmap Write shader evaluates the object's render tree directly. Whatever portion of the render tree is connected to the Lightmap Write's input port is computed and written to a texture file.
Any illumination shader can be used to sample the lighting in the model for the lightmap. For example, you can use a Lambert or even experiment with a Phong and the scattering of specular reflections.
You must manually generate the lightmap image for fast subsurface scattering effects created when using the pre-packaged Fast Subsurface Scattering Shader Compounds [Materials and Shaders].