how do I use a texture as a mask?

So I want 95% of a actor to be reflective, and his face to not have the reflection texture

I have a black and white mask I made, how do I use it?

Make sure its before the texture(s) you want it to affect, then make sure RGB to intensity and stencil are enabled. either black or white should be fully reflective or not, you can change it by toggling negative :smiley: