The IfcRelInterferesElements objectified relationship indicates that two elements interfere. It is a 1 to 1 relationship, and the concept of two elements interfering physically or logically is described independently of the elements. 

The interference may be related to the shape representation of the entities by providing an interference geometry or zone:

• when the interference is between two physical products, the InterferenceGeometry attribute is used to define the physical interference shape, it can be part of the shape of one of the elements used in the relationship or define the interface between the two shapes using a IfcConnectionGeometry.
• when the interference is between two spatial objects, the InterferenceSpace attribute is used to define the interference space between the two footprints associated to the spatial objects, expressed by a specific IfcSpatialZone of predefined type IfcSpatialZoneTypeEnum INTERFERENCE.

If the interference geometry and Zone are omitted then the interference is provided as a logical relationship. Under this circumstance, the connection point, curve, surface, or solid has to be recalculated by the receiving application. Both InterferenceGeometry and InterferenceSpace should not be set together. 

The RelatingElement and RelatedElement define the two elements in the relationship, that may have different roles. The relation orientation may be required by certain InterferenceType values or InterferenceGeometry calculation, this is done by setting the attribute ImpliedOrder accordingly:

• ImpliedOrder=TRUE The RelatingElement constitutes the primary element of the interference relationship that is oriented from RelatingElement (source) to RelatedElement (target). If the interference is to be resolved by subtracting the overlapping part, it should be subtracted from the RelatingElement. The net result would be the RelatingElement subtracted by the InterferenceGeometry. This would be the case in interference relationships where the RelatedElement creates a void in the RelatingElement dynamically.
• ImpliedOrder=FALSE The RelatingElement and RelatedElement have no priority among each other. If the interference is to be resolved then no information about whether the InterferenceGeometry should be subtracted from the RelatingElement or the RelatedElement can be traced. This would be the case for clash detection results.
• ImpliedOrder=UNKNOWN No information about the priorities is provided.

The InterferenceType property optionally specifies the type of interference between the two elements, two set of default types are provided:

• Oriented interferences types imply usage of ImpliedOrder set to TRUE and specific choice of RelatingElement and RelatedElement to be meaningful:
  • Crosses: the RelatingElement is crossing the RelatedElement (e.g. Railway crossing a road)
  • PassesThrough: the RelatingElement is passing through the RelatedElement (e.g. a Road passing inside a tunnel)
  • PassesOver: the RelatingElement is passing over the RelatedElement (e.g a bridge passing over a water canal)
  • PassesUnder: the RelatingElement is passing under the RelatedElement (e.g a Tunnel passing under a road)
• Non oriented interferences types do not imply specific values of ImpliedOrder (but can still be set to detail shape interference calculation)
  • Clash: The RelatingElement and RelatedElement have a spatial or shape-based clash
  • Along: The RelatingElement and RelatedElement have a common frontier/surface

changes 

• IFC4 changes:
  • Type of the ImpliedOrder attribute changed from logical to IfcLogical.
  • Type of the RelatedElement attribute changed from IfcElement to IfcInterferenceSelect.
  • Type of the RelatingElement attribute changed from IfcElement to IfcInterferenceSelect.