A feature element addition is a specialization of the general feature element, that represents an existence dependent element which modifies the shape and appearance of the associated master element. The IfcFeatureElementAddition offers the ability to handle shape modifiers as semantic objects within the IFC object model that add to the shape of the master element. 

The IfcFeatureElementAddition is associated to its master element by virtue of the objectified relationship IfcRelProjectsElement. This relationship implies a Boolean 'union' operation between the shape of the master element and the shape of the addition feature. The local placement for IfcFeatureElementAddition is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which defines the local coordinate system that is referenced by all geometric representations. The local placement is always defined in relation to the local placement of the element to which the feature element is added:

• The PlacementRelTo attribute of IfcObjectPlacement points (if specified) to the object placement of the IfcElement, which is used in the ProjectsElements.RelatingElement inverse attribute (the parent element of the feature).

• This definition cannot be instantiated.