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