Starting with DocBook V5.0, DocBook is identified by its namespace, http://docbook.org/ns/docbook. The particular version of DocBook to which an element conforms is identified by its version attribute. If the element does not specify a version, the version of the closest ancestor DocBook element that does specify a version is assumed. The version attribute is required on the root DocBook element.

Here is how these attributes would appear on the book element.

<book xmlns="http://docbook.org/ns/docbook"
      version="5.0">
  …
</book>

If you change the DocBook schema, the namespace remains the same, but you must provide an alternate version identifier for the schema and the modules you changed. The version attribute identifies the version of DocBook the alternate is based on, specifies what type of variant it is, and names the variant and any additional modules. While the format for the version string is not part of the normative specification, the DocBook Technical Committee recommends the following format:

base_version-(subset|extension|variant) (name[-version])+

For example, version 1.0 of Acme Corporation’s extension of DocBook V5.0 could be identified as “5.0-extension acme-1.0”.

If your schema is a proper subset, use the subset keyword in the version. If your schema extends the markup model, use the extension keyword. If you’d rather not characterize your variant specifically as a subset or an extension, use the variant keyword.

Although not directly supported by RELAX NG, in some cases it may still be valuable to identify a DocBook V5.0 customization layer with a public identifier. A public identifier for DocBook V5.0 is:

-//OASIS//DTD DocBook V5.0//EN

If you make any changes to the structure of the schema, you must change the public identifier. You should change both the owner identifier and the description. Formal public identifiers for the base DocBook modules would have identifiers with the following syntax:

-//OASIS//text-class DocBook description Vversion//EN

Your public identifiers should use the following syntax:

-//Owner-ID//text-class DocBook Vversion-Based (Subset|Extension|Variant) 
Description-and-version//lang

For example:

-//O'Reilly//DTD DocBook V5.0-Based Subset V1.1//EN

If your schema is a proper subset, use the Subset keyword in the description. If your schema extends the markup model, use the Extension keyword. If you’d rather not characterize your variant specifically as a subset or an extension, use the Variant keyword.