DocBook is a very popular set of tags for describing books, articles, and other prose documents, particularly technical documentation.
A file is considered to be a DocBook 4 document when one of the following conditions are true:
The schema of DocBook 4 documents is ${frameworks}/docbook/dtd/docbookx.dtd, where ${frameworks} is a subdirectory of the Oxygen XML Developer install directory.
The XML catalog is stored in ${frameworks}/docbook/catalog.xml.