IfcBuildingElementProxy (in IFC 4 also IfcBuiltElement proxies) is a placeholder class for elements that do not fit any specific IFC entity. It is legitimate for genuinely unusual objects.
In practice, large numbers of proxies usually mean the authoring tool did not know how to map its categories to IFC. Proxy elements are hard to filter, check or measure, so model checks often count them.
Discussion
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