The term “chapter file” as used here applies to files that contain chapters, but also to those that contain a preface or appendix. All appear at the same level in a table of contents.

Chapter files have an XML declaration and a DOCTYPE declaration that contains references to XML entity files, just like the main document. The root element should be appropriate for the file content (either <preface>, <chapter>, or <appendix>), and the root element named in the DOCTYPE must match. For example, a file that contains an appendix might begin like this:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd"
[
... references to entity files ...
]>
<appendix id="restrictions">
...

The file then continues with its title and all the other elements that make up its content (paragraphs, tables, lists, and so forth).

All levels below the preface/chapter/appendix level are sections, written using <section> elements. Nested sections also use <section>. Here is an example of how to nest sections within a chapter:

<chapter>
<section>
(This is a section)
<section>
 (This is a subsection)
 <section>
  (This is a subsubsection)
 </section>
</section>
</section>
<section>
(This is a section)
</section>
</chapter>

We don't use the <sect1> through <sect6> elements because <section> can be handled more flexibly. For example, it's easier to promote or demote a section a level when all sections use the <section> tag rather than numbered section tags. Also, there is no limit to the depth of <section> element nesting.

Chapter files are “standalone.” That is, they are well-formed XML files, even if not included into the main document. Among other things, this means you can validate a chapter file separately, and you might generate output using just a particular chapter file (not the entire document). For example, we could create a PDF version of just the Cluster chapter from the Reference Manual.