Material Input Channels
The SubsurfaceColor input channel defines the color inside the material which affects SSS lighting.
The Opacity channel of the material takes on a slightly different meaning when the material is using subsurface scattering and the MLM_Subsurface shading model. Since these types of surfaces are completely opaque, the Opacity, in this case, controls how dense the material is when it scatters light as well as:
How much the normal affects the subsurface lighting, a more opaque material gets more normal influence.
How far lighting makes it through the material due to self-shadowing, a smaller opacity causes light to travel further.
How soft the shadow is on the material, a smaller opacity results in softer shadows, but softness is clamped.
Be sure to set the Opacity for any subsurface materials to a reasonable value, e.g. 0.1. The default opacity is 1, which does not produce a very convincing subsurface-type effect.