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.