The identifier of an object within a document (table, figure, sidebar, etc.) that is the target of the <related-object> element
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 ID 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 Identifier attribute. If an entire deliverable unit is the destination, such as a link to an entire chapter or journal article, that unit is specified by the Document Identifier attribute, and the Object Identifier attribute is omitted.