source-id 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.
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.