The submitting working group discussed whether or not to make the documentURI element of type sml:refType (an SML reference) instead of making it of type xs:anyURI as is done in the submission.  The group at that time saw no clear need for an SML reference in this context, wanted to avoid what was seen as unnecessary complexity, and noted that locatorType (which contains the element declaration for documentURI) includes an xs:any and xs:anyAttribute for extensibility, so it could support SML references in locators without schema updates using any scheme the implementation supports.