Each chapter starts with a <chapt> tag, followed by the chapter's title. The title may contain marked-up inline text, but no cross-references (see Marked-up inline text and character style markup, Chapter 4). The same applies for an appendix, except that it starts with a <appendix> tag.
The body of the chapter or an appendix is zero or more paragraphs, the first of which must be indicated by a <p> tag to distinguish it from the title, and then zero or more sections.
A section starts with <sect>, and has a similar structure: title, optionally some paragraphs, and then optionally some subsections.
Subsections are <sect1>; there are also smaller divisions <sect2>, <sect3> and <sect4>.
Paragraphs are introduced by <p>. Sometimes the start of paragraph tag can be omitted, but it is mandatory after <chapt>, <sect> and so forth. It is never necessary to mark the end of a paragraph with </p>.
Paragraphs can contain marked up inline text (see Marked-up inline text and character style markup, Chapter 4) and also lists and examples (Lists and examples, Chapter 5).
ardo@debian.org
ijackson@gnu.ai.mit.edu