Proposal to formalise how property set templates are applicable to external classification systems

Resurrecting and taking a fresh look at @jmirtsch 's question [here](https://forums.buildingsmart.org/t/property-set-template-applicable-entities-by-external-classification/199). Here is my paraphrase:

Currently, `IfcPropertySetTemplate` has an `ApplicableEntity` attribute which denotes whether or not it is applicable to a particular IFC class and predefined type. However, what if a user wants to say that a property set template is applicable to an element classified using an external classification system, such as Uniclass / Omniclass / etc? @jmirtsch proposes to use the existing `HasAssociation` attribute.

This has no change to the schema, but may require an additional section in the documentation describing that this relationship can be used this way.

Thoughts? Also, does recent developments with the bSDD impact this proposal in any way?

Personally, I think the proposal makes sense.

Ping @TLiebich @berlotti @aothms

Read the linked buildingSMART forum discussion and the IFC4.x documentation for IfcPropertySetTemplate, especially ApplicableEntity and HasAssociation. Clarify whether HasAssociation should express applicability to external classification systems such as Uniclass or Omniclass, and assess the bSDD implications. Done means reaching agreement and documenting the supported relationship without changing the schema.

Written by the indexing model from the issue text.