PBR Material, what is going in the thicknessmap.b channel?

What is going in the PBR materials thicknessmap.b channel? It is mentioned in documentation that is reserved, but not what for.

Can additional arbitrary channels beyond RGB be added to a texture file and read? I have seen it in other engines where you can sometimes use alpha or extra channels in DDS and other format textures. For optimization it cuts down on file size a bit vs starting a new texture file.

Pinging @sebavan

In the thickness map, r is the thickness factor, g can be used to pack the refractionIntensity, b the translucencyIntensity and a the scatteringIntensity in prevision of scattering support.