Using of Pset_ElementSize

Hello everyone! I want to use in my model software standardized PropertySet Element Size ( Pset_ElementSize - IFC4.3.2.0 Documentation) for IfcController elements. But I found very amazing occasion. This Pset recommended to subtypes of IfcFlowController, but not to IfcController.
Why was this done? Why this Pset_ was oriented only for distinguished band of the classes?