The IfcElementAssembly represents complex element assemblies aggregated from several elements, such as discrete elements, building elements, or other elements. The geometry of an IfcElementAssembly is generally formed from its components, in which case it does not need to have an explicit geometric representation. In some cases it can be useful to also expose an own explicit representation of the aggregate. 

The IfcElementAssembly represents an aggregate, i.e. it should have other elements of IfcElement subtypes as contained (sub)parts. 

• The IfcElementAssembly is an aggregate i.e. being composed by other elements and acting as an assembly using the objectified relationship IfcRelAggregates, referring to it by its inverse attribute SELFIfcObjectDefinition.IsDecomposedBy. Components of an assembly are described by instances of subtypes of IfcElement.
• In this case, the contained subtypes of IfcElement is not additionally contained in the project spatial hierarchy, i.e. the inverse attribute SELFIfcElement.ContainedInStructure of those IfcElement's should be NIL.

The image below illustrates spatial containment and element aggregation relationships: 

The Spatial Containment concept template applies to this entity as shown in the table below:

The IfcElementAssembly should have a relationship for its containment in the hierarchical spatial structure of the project. Only if the IfcElementAssembly is itself a part of another assembly this relationship should be omitted.

• The IfcElementAssembly is a general purpose entity that is required to be decomposed. Also other subtypes of IfcElement can be decomposed. The assembly structure can be nested, i.e. an IfcElementAssembly could be an aggregated part within another IfcElementAssembly.
• View definitions and/or implementer agreements can restrict the number of allowed levels of nesting.
• View definitions or implementer agreements can further constrain the applicability of certain shape representations at the IfcElementAssembly in respect of the shape representations of its parts.

changes 

• IFC4 changes: Attribute PredefinedType is optional.