This term is related to the material that covers most of the exterior walls of the building. If two materials each cover equal areas of walls, choose the heavier material. Where there are multiple materials present on the exterior walls, choose the material that covers more of the walls by area than any other material.
In some cases this will be the same material as "Material of the lateral load resisting system" if the lateral loads are resisted by the exterior walls and if the structural material is exposed. However in many cases the material that covers the exterior walls will be a separate cladding material that does not resist lateral loads and is laid over the lateral load-resisting structure. In both cases, the material of exterior walls should still be defined if it is known, even if it is the same material that forms the lateral load-resisting structure.