From the pattern: "Wherever possible, define the format using existing standards, referencing their
elements by namespace rather than rolling your own. For example, add metadata to your metadata using
RDF and the Dublin Core extensions rather than inventing your own <author> and <description> tags.
Allows for independent evolution of markup by parties who know the business domain best."
This pattern is from Kyle Downey's article Architectural Design Patterns for XML Documents.
The Composition Pattern is a sub set of the Reuse Document Type Pattern