The IfcFeatureElementSubtraction is specialization of the general feature element, that represents an existence dependent elements which modifies the shape and appearance of the associated master element. The IfcFeatureElementSubtraction offers the ability to handle shape modifiers as semantic objects within the IFC object model that subtract from the shape of the master element. 

The voiding relationship between a master element and a subtraction feature is geometrically resolved by a Boolean difference operation. The local placement for IfcFeatureElementSubtraction 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 building element from which the feature element substration is substracted:

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

• This definition cannot be instantiated.
• A single subtraction feature such as the subtype IfcOpeningElement is assigned by a single subtraction relationship IfcRelVoidsElement to one occurrences of IfcElement. It establishes a 1:1 relationship between the opening and the element. An element can have several IfcRelVoidsElement relationships, enabling several voids.