Openings are one the most complex elements when coordinating designs between architects, structural engineers, MEP engineers and construction companies (one would not believe that).
So how do openings work in IFC?
The IfcOpeningElement class is here to cut voids into other elements. So you can create a geometry (if possible it should be made of extrusions perpendicular to the element to be cut). Within the IFC file a relationship is established between the cutting and the cut elements.
That means an opening has its own identity and life cycle. It can but does not necessarily need to be filled with other elements like a door or a window.
If you compare this to other BIM software they usually work differently. There the opening is part of the window or door element.
Why is the IFC approach more elegant? Say you have a wall hosting a door (existing building). You want to remove the door. In Revit, for example, the wall is automatically filled with the original wall. But is that realistic? In IFC you can remove the door but still keep the opening. Then you can fill it with whatever you want: a new door, a window, or even a new wall or a shelf (furniture).
So it’s always a host being voided by zero to many openings each being filled by zero to many another element.
That makes sense, don’t you think?