document-id Document Identifier

Identifier of a document within a publication that is (or contains) the target of the <related-object> element.

Usage/Remarks

This attribute holds an external identifier assigned to a publication that is part of the description of a related object (<related-object>). The contents of this attribute should not be confused with the @id attribute, which holds an internal document identifier that can be used by software to preform a simple link.
OPTIONAL on element: <related-object>
Value Meaning
Text, numbers, or special characters An identifier for the document that contains the related object (<related-object>).
Restriction @document-id is an optional attribute; there is no default.