Information about the materials assigned to a StaticMesh LOD. The StaticMesh stores a Material List and per-mesh-Section assignments into that Material List. This relationship is not necessarily 1-1, multiple Sections can use the same material, and there can be arbitrary remappings, and the storage of this information is somewhat scattered across StaticMesh. This data structure encapsulates the critical information needed to (eg) assign materials to a new mesh/component such that they match the rendered materials on an existing StaticMesh.