Setting a material to subsurface profile causes it to stop respecting the opacity value when applying subsurface scattering.
Create a subsurface profile material with a valid subsurface profile.
Assign to a mesh and place the mesh under a bright point light
Assign a constant of 0 to the opacity input of the material, and note the appearance of the mesh.
Assign a constant of 1 to the opacity input of the material, and note the appearance of the mesh.
There is no difference.
Because of this bug, it is impossible to mask the subsurface effect to limit it to only parts of the material.