Identifier of an object (for example, a table, figure, or sidebar) within a separate document that is the target of the <related-object> element.
Remarks
Used along with @object-id-type to identify the object within a document that is the target of the <related-object> element. The @object-id might contain a DOI as content, and the @object-id-type should indicate that this identifier is a DOI. The @object-id also might contain the XML ID of the target object.
This attribute is used to identify a specific location within a larger deliverable unit, for example, a paragraph or figure
within a chapter. The deliverable unit is specified by the @document-id attribute. If an entire deliverable unit is the destination, such as a link to an entire chapter or a complete journal article,
that unit is specified by the @document-id attribute, and the @object-id attribute is omitted.