It seems like without a type, it is a sub-optimal entity type to use for mapping objects that have both instance and type properties.
Can you give an example where a IfcProjectElementType should be used?
What are instance-entities without a type-entity? IfcWall?
It should be “IfcProjectionElementType”. I corrected the error in the topic title.
This question comes from a discussion in the Autodesk revit-ifc GitHub repository about the best default for classifying/mapping Revit Wall Sweeps to IFC. The user can change the default but have a good default might lessen the burden on the user.
Plus, the “old way” of doing things like this through “IfcBuildingElementProxy” as a catch-all is being discouraged (or is not appreciated by the users), with more classes being better defined and related to the desired semantic relationships like IfcProjectionElement or IfcBuildingElementPart.
In this case, the debate is whether IfcProjectionElement or IfcBuildingElementPart is better. IfcProjectionElement looks like it has a lot of potential for this case, but it does NOT have a corresponding Type class that would help when developing sweeps for things like brick ledges, corbels, stone caps, etc., that are integral parts of the wall definition, but have their own characteristics.
Projections sound to me as an oddity in the classification system. Everything is a “thing” with a function, whereas projection simply describes a geometric characteristic. (in contrast with openings, which are semantically penetrations with requirements).
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@Moult Would you consider IfcFeatureElementAddition and IfcFeatureElementSubtraction similar oddities?
Same feeling as @Moult about “Projections” for that purpose.
It feels a little weird right now. For subtractions, there are opening penetrations are for construction and installation with requirements, earthworks which have semantics for excavation, and voiding features which have semantics for fabrication (drilling, milling, punching, etc).
For projections however, there just seem to be blisters and deviators. I’m agreeing with you that it seems a little underdeveloped compared to subtractions. If it were further fleshed out as you say with corbels, ledges, haunches, turndowns, earth fill, then it would have more merit.
I’m no expert in classification systems though so all this is just gut feel.