The mask, in this case, could be an SSS mask to dictate where the shader applies a sub-surface effect. Or a facial hair mask/etc. If the diffuse texture had an alpha channel(which it wouldn't for this purpose as it would destroy mipping) then it wouldn't be masking inside the bounds of the texture but, around it. If that…
There were 3 other textures along with this one: - Diffuse (base colors/details) - Specular Map (B/W image) - Normal Map I took a look at the channels, R and G have two different Greyscale textures, and Blue is Black, so it seems it's as you say. I'm curious how this would work for masking, why not just utilize the Diffuse…