Theory of SSS

Basically, the directly reflected light from the object surfaces is defined via the Roughness and Metallic textures. This mechanism is sufficient for rendering most of objects.

The SSS (sub-surface scattering) effect, however, is a mechanism of light transport. The light pass through the surface of a translucent object, interact with the insides of the material at irregular angles, and then exits the surface and scatters. Please note that the SSS effect includes the SSS Light and SSS Transmission Lights.

Therefore, the human skin's actual look should be not only the directly reflected lights, but also the blending of the sub-suface scattering lights.