document-link
A categorized link to an external document.
| @url | The target URL of the external document, e.g. "http://www.example.org/doc.html". |
| @format | The MIME type of the external document, e.g. "application/pdf". A partial list of MIME types appears at http://iatistandard.org/codelists/file_format |
| @language | The ISO 639 language code for the target document, e.g. "en". |
