IFC 4.3.0.1 (IFC4X3_TC1)

4.4 Object Composition

Objects may be composed into parts to indicate levels of detail, such as a building having multiple storeys, a framed wall having studs, or a task having subtasks. Composition may form a hierarchy of multiple levels, where an object must have a single parent, or if a top-level object then declared within the single project or a project library.