Material Editor > Maps rollout > Click a Map button. > Material/Map Browser > Mask
With the Mask map, you can view one material through another on the surface. The mask controls where a second map is applied to the surface.
By default, lighter (whiter) areas of the mask are opaque, showing the map. Darker (blacker) areas of the mask are transparent, showing the underlying material. You can use Invert Mask to reverse the mask's effect.