reference — Describes an external document
reference : xmlns:xlink, xlink:type, refID, author, title, [classification], [accession], date, [xlink:href]
description?
xmlns:xlinkxlink:typerefID- an internal, document-unique, XML identifier for this reference definition
author- the name of the author of the reference
title- the title of the referenced document
classification(optional) - the security classification of the document
accession(optional) - the accession number (ISBN or organization report number) of the document
date- the date of the document, in ISO 8601 (YYYY-MM-DD) format
xlink:href(optional) - an optional URL to an on-line copy of the document