The nesting relationship IfcRelNests is a special type of the general composition/decomposition (or whole/part) relationship IfcRelDecomposes. The nesting relationship can be applied to all non physical subtypes of object and object types, namely processes, controls (like cost items), and resources. It can also be applied to physical subtypes of object and object types, namely elements having ports. The nesting implies an order among the nested parts.

• EXAMPLE: A nesting of costs items in an IfcCostSchedule is the composition of complex cost items from other cost items. The order of the nested cost items underneath the parent cost item is determined by the order of the list of RelatedObjects.
• EXAMPLE: A nesting of IfcTask's within a work schedule is the composition of a parent work task from more specific sub work tasks. The order of the sub tasks underneath the parent task is determined by the order of the list of RelatedObjects.
• EXAMPLE: A series of IfcDistributionPort's can be nested within an IfcDistributionElement. They decompose the distribution element and have an implied order.

changes 

• IFC4 changes: The attributes RelatingObject and RelatedObjects are demoted from the supertype IfcRelDecomposes, and RelatedObjects is refined to be a list.

The use of IfcRelNests is re-purposed to be a nesting of an ordered collections of parts.