Source Identifier (in Related Object)
Identifier of a journal, book, conference proceedings, or other publication that is (or contains) the target of the <related-object>.
Used along with @source-id-type to identify the publication that is (or contains) the target of the <related-object> element. The @source-id might contain an ISBN as content, and the @source-id-type should indicate that this identifier is an ISBN.
Best Practice: For each source, document, and object, an identifier is specified on the corresponding attribute. If the source and document are the same, the @source-id attribute should be empty. If the document and object are the same, the @object-id attribute should be empty. The @document-id attribute should always be specified.