A file is considered to be a DocBook 5 document when the namespace is http://docbook.org/ns/docbook.
The default schema that is used if one is not detected in the DocBook 5 file is docbookxi.rng and it is stored in [OXYGEN_DIR]/frameworks/docbook/5.0/rng/. Other types of schemas are also located in various folders inside the [OXYGEN_DIR]/frameworks/docbook/5.0/ directory (for example, XML Schema files are located in the xsd folder).
The default CSS files used for rendering DocBook content in Author mode is stored in [OXYGEN_DIR]/frameworks/docbook/css/.
The default XML catalog, catalog.xml, is stored in [OXYGEN_DIR]/frameworks/docbook/5.0/.
To watch our video demonstration about editing DocBook documents, go to http://oxygenxml.com/demo/DocBook_Editing_in_Author.html.