The type of document identifier, such as DOI, publisher’s identifier, etc.
Used along with @document-id to identify the document within a publication that is (or contains) the target of the <related-object> element. The @document-id might contain a DOI as content, and the @document-id-type should indicate that this ID is a DOI.
|Text, numbers, or special characters||The type of document identifier involved for a related object<related-object>|
|Restriction: This attribute may be specified if the element is used.|