The decomposition relationship, IfcRelDecomposes, defines the general concept of elements being composed or decomposed. The decomposition relationship denotes a whole/part hierarchy with the ability to navigate from the whole (the composition) to the parts and vice versa. Decompositions may be constrained by requiring both, the whole and its parts, to be of the same type - thus establishing a nesting relationship. Or they may require some form of physical containment, thus establishing special types of aggregation relationships. 

• EXAMPLE: A cost element is a nest of other cost elements. Or a structural frame is an aggregation of beams and columns. Both are applications of decomposition relationship.

Decompositions imply a dependency, i.e. the definition of the whole depends on the definition of the parts and the parts depend on the existence of the whole. The decomposition relationship can be applied in a recursive manner, i.e. a decomposed element can be part in another decomposition. Cyclic references have to be prevented at application level.

There are two special names for decomposition, which are linguistically distinguished, nesting and aggregation. The subtypes of IfcRelDecomposes will introduce either the nesting or aggregation convention (see IfcRelNests and IfcRelAggregates). 

changes 

• IFC4 changes:
  • The differentiation between the aggregation and nesting is determined to be a non-ordered or an ordered collection of parts.
  • The attributes RelatingObject and RelatedObjects have been demoted to the subtypes.