The 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 ID is an ISBN.
For a large source (such as a book) it may be desirable for the link to cause delivery of only a certain portion (such as a chapter). In that case, the book is specified by this attribute, while the particular chapter is specified by the @document-id attribute. If the entire destination is delivered, it should be specified using the @document-id attribute, and this attribute should be omitted.