The Mask Mesh should contain any geometry within the scene that occludes the subject from a given camera view.
In this example, the plinth occludes the shoe for the lower cameras.
If the plinth is not included within the scene, parts of the plinth will be mistakenly projected onto the underside of the shoe
To avoid this, use a separate ‘Mask Mesh' to denote unwanted geometry that occludes the subject, in this case, the plinth. Blue will be used for the final texture, Red will not.
Providing a mask mesh ensures that these areas will not affect the final asset, as otherwise they can be projected across the surface, leading to problems in the final textures.