A customer presented a server to me that has used an (abstract) HasChild reference from a FolderType to a Variable. I pointed them to parts of the specification that this violates, namely:

• https://reference.opcfoundation.org/Core/Part3/v104/docs/7.5 : "The HasChild ReferenceType is an abstract ReferenceType; only subtypes of it can be used. "
  and
• https://reference.opcfoundation.org/Core/Part3/v104/docs/5.3.2 : "The IsAbstract Attribute indicates if the ReferenceType is abstract. Abstract ReferenceTypes cannot be instantiated ..."

However, they refused to acknowledge that there is any problem on their side. It is hard to grasp why, but I could not win the argument because

1) The wording used in the specification uses undefined terms: It is not defined what "used" means in "only subtypes of it can be used", and it is not defined what "instantiated" means in "Abstract ReferenceTypes cannot be instantiated ".
2) There is no "shall" or "shall not".

I think that we should:
1) Define what "instantiation" means. It actually means different things for different entities. For ReferenceTypes, I think that the ReferenceType is instantiated whenever it appears in BrowseResult.References[].ReferenceTypeId returned from the Browse or BrowseNext service call, because that is the way clients can actually observe it.
2) We should change the wording to say "Abstract ReferenceTypes shall not be instantiated ..."