Setting CanApply to false will prevent the Material Tool from applying materials to any SkinnedMeshRenderer/MeshRenderer components in the slot.
Examples
On an avatar, you can try adding this component to the Slot of each SkinnedMeshRender, setting CanApply to false in each case. Doing this will ensure that the avatar's material cannot be changed by the Material Tool.
Adding this component to the Slot of a MeshRender and setting CanApply to true can allow one to explicitly change the material in specific places on small objects.