A system family in Revit built from stacked layers (structure, insulation, finishes) that hosts doors, windows, and other openings. A wall's layers - each with its own material, thickness, and function (structure, thermal, finish) - are defined once in a type and reused across every instance of that type, so editing the type's layer stack updates every wall using it at once. Walls automatically join at corners and clean up intersections with other walls, and everything from doors to curtain panels hosts to them.