Hi Brian,
It's a matter of configuring Windchill whether it allows you to choose the type of the link or not regardless the type of WTDocument. OOTB it doesn't allow you to choose. We changed our Windchill and removed that limitation, we are happy with it. See Preference Management > Part Management > Part to Document Association Logic.
Regards, Hugo.
Quote from the help:
Controls the logic for how documents are related to and displayed for a part. A value of false uses the PDMLink logic and a value of true uses the Windchill PDM logic. The PDMLink logic enforces that a document of type Reference document can only be associated to the part as a References document and all other document types can only be associated to a part as a
Described By document. The Windchill PDM logic allows References and Described By relationships to be created from part to document regardless of the document type. See the References and Described By tables on the part information page for a description of the behavior of References documents and Described By documents. In addition, the PDMLink logic enforces that a part be related only to one version of a document; whereas the Windchill PDM logic allows a part to be associated to versions of the same document.