A virtual element is a special element used to provide imaginary boundaries, such as between two adjacent, but not separated, spaces. Virtual elements are usually not displayed and does not have quantities and other measures. Therefore IfcVirtualElement does not have material information and quantities attached. 

Uses of IfcVirtualElement include:

• An imaginary boundary such as between two adjacent, but not separated, spaces. The IfcVirtualElement can provide the 2D curve or 3D surface representation of the virtual space connection and is referenced by two instances of IfcRelSpaceBoundary, each pointing to one of the two adjacent IfcSpaces. The boundary is exchanged using PredefinedType = BOUNDARY.
• A provision for voids. Such as where a particular volume of space is requested by an engineering function that might later be accepted or rejected. If accepted it is transformed into a void within a building element, like a wall opening, or a slab opening. The provision for voids is exchanged using the PredefinedType = PROVISIONFORVOID. Typically, it uses a swept solid geometry, where the profile of the swept solid lies on/near the surface of the referred building element and the extrusion depths is equal to or bigger than (in case of round or otherwise irregular element shape) the thickness of the building element. The appropriate Pset_ProvisionForVoid property set should be attached.
• A clearance space. Typically, a placeholder for the necessary space allocation for future designed elements, such as mechanical equipment that will be determined in a later design phase. The clearance is exchanged using the PredefinedType = CLEARANCE.

The image below describes how to use IfcRelSpaceBoundary in conjunction with IfcVirtualElement to define space boundaries. 

changes 

• IFC4X3 changes: Virtual elements can now be used for void provisions and clearance to replace existing functionality on IfcBuildingElementProxy.