The layered assembly definition (structure, insulation, finish, etc.) behind a system family like a wall, floor, or roof - each layer carrying its own material, thickness, and function. Editing a wall type's compound structure is how a modeler defines what it's actually made of layer by layer - which is what drives its fire rating, thermal performance, and the material quantities that flow into schedules, rather than the wall just being a single generic slab with no real construction logic behind it.