A file is considered to be a DocBook 5 document when the namespace is http://docbook.org/ns/docbook.
DocBook 5 documents use a Relax NG and Schematron schema located in ${frameworks}/docbook/5.0/rng/docbookxi.rng, where ${frameworks} is a subdirectory of the Oxygen XML Author install directory.
The CSS file used for rendering DocBook content is located in ${frameworks}/docbook/css/docbook.css.
The XML catalog is stored in ${frameworks}/docbook/5.0/catalog.xml.