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 subtypes of object and object types. For example, processes, controls (like cost items), and resources. It can also be applied to alignment, nesting its different layouts; and to physical subtypes of object and object types, such as elements having ports. The nesting implies an order among the nested parts.
EXAMPLE A nesting of cost 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 entities 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 entities can be nested within an IfcDistributionElement. They decompose the distribution element and have an implied order.
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 behaviour that is implied from the dependency has to be established inside the applications.
HISTORY New entity in IFC2.0
{ .change-ifc2x4}
IFC4 CHANGE The attributes RelatingObject and RelatedObjects are demoted from the supertype IfcRelDecomposes, and RelatedObjects is refined to be a list. The use of IfcRelNests is repurposed to be a nesting of an ordered collections of parts.
The object definition, either an object type or a object occurrence, that represents the nest. It is the whole within the whole/part relationship.
{ .change-ifc2x4}
IFC4 CHANGE The attribute has been demoted from the supertype IfcRelDecomposes and defines the ordered nesting relationship.
The object definitions, either object type or object occurrence, that are being nested. They are defined as the parts in the ordered whole/part relationship - i.e. there is an implied order among the parts expressed by the position within the list of RelatedObjects.
{ .change-ifc2x4}
IFC4 CHANGE The attribute has been demoted from the supertype IfcRelDecomposes and defines the ordered set of parts within the nest.
The instance to which the relation points as provided by RelatingObject shall not be contained in the list of RelatedObjects.