Document Component Identifier
Identifier of a major document component within a publication that is (or contains) the target of the <related-object> element.
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.
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.