The type of object identifier, such as an XML ID, a string, the label of the object, etc., for a <related-object> element
Used along with @object-id 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.
|Text, numbers, or special characters
|The type of the identifier, for example, “ID” for an XML attribute of type ID
|Restriction: This attribute may be specified if the element is used.