External Identifier: This element holds an external identifier assigned to a publication that is part of the description of a related object (<related-object>). The contents of this attribute should not be confused with the @id attribute, which holds an internal document identifier that can be used by software to preform a simple link.

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 should be specified on the corresponding attribute. For any related-object link, the @source-id attribute points to the largest publishable unit, for example, an entire book. The @document-id attribute points to a major component of the source; for example a chapter, front matter section (Preface) or back matter section (Appendix) of a book. Use the @object-id attribute to point to any item within the specified major document component; for example a section, figure, table, or reference within the chapter of the book.

Where the more specific attributes are populated, the less specific attributes should also be populated. For example, if @object-id is specified, then both @document-id and @source-id should also be populated. If @source-id points to the intended target (an entire book), then neither @document-id nor @object-id should be specified.

The <related-object> element can also provide a name for the type of each identifier used, for example, ISBN, ISSN, DOI, XMLID, URI, and so on. These go on @source-id-type, @document-id-type, and @object-id-type as appropriate.