Physical building elements include IfcWall, IfcBeam, IfcDoor, IfcWindow, IfcStair, etc. Distribution elements (HVAC, electrical, plumbing) have a concept of ports where elements may have specific connections for various services, and connected together using cables, pipes, or ducts to form a system. Various connectivity relationships are used for building elements such as walls having openings filled by doors or windows.