Definition from IAI: 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 constraint 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.

NOTE: 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).
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.

HISTORY New entity in IFC Release 1.5, it is a generalisation of the IFC2.0 entity IfcRelNests.
ISSUE See issue log for changes made in IFC Release 1.5.1. See issue and change log for changes made in IFC Release 2x.

EXPRESS specification:

ENTITY IfcRelDecomposes
ABSTRACT SUPERTYPE OF (ONEOF(IfcRelAggregates, IfcRelNests))
SUBTYPE OF ( IfcRelationship);
RelatingObject  :  IfcObject;
RelatedObjects  :  SET [1:?] OF IfcObject;
WR1  :  SIZEOF(QUERY(Temp <* RelatedObjects | RelatingObject :=: Temp)) = 0;

Attribute definitions:

RelatingObject  :  The object that represents the nest or aggregation.
RelatedObjects  :  The objects being nested or aggregated.

Formal Propositions:

WR1  :  The instance to which the relation RelatingObject points shall not be contained in the List of RelatedObjects.

References (5):

Name Type Referred through Express-G
IfcObject Entity
Attribute 'Decomposes'
Attribute 'IsDecomposedBy'
Diagram 1
IfcRelAggregates Entity
Diagram 3
IfcRelNests Entity
Diagram 3
IfcRelationship Entity
Diagram 1

Inheritance graph

ENTITY IfcRelDecomposes;
GlobalId  :  IfcGloballyUniqueId;
OwnerHistory  :  IfcOwnerHistory;
Name  :  OPTIONAL IfcLabel;
Description  :  OPTIONAL IfcText;
ENTITY IfcRelationship;
ENTITY IfcRelDecomposes;
RelatingObject  :  IfcObject;
RelatedObjects  :  SET [1:?] OF IfcObject;