This is the name of the image file containing the mask to block out areas not wanted in a photo-texture.
In a multi-photo-texture, the areas masked in one image will be textured by that same area found in other images, where available. In a single photo-texture, the areas masked will appear grey. This prevents obstructions or occlusions from appearing in the photo-texture.
If the mask is partial (i.e. pixel values between white and black or 0 and 255), blending will occur.
Note: The masked photo-texture will only take effect in the 3D Viewer and export when Quality textures are enabled.
See also Creating mask images and Create Masks Dialog.