<sn [id=name]>heading topic text...
Where n is the level number (1, 2,..., or 9).
Topics entered with <chapter> can have subtopics entered with <s1>, <s1> topics can have <s2> subtopics, and so on. You cannot skip a level.
The heading for a section can be on the same line as the <sn> tag or on the next line; a heading is required. Text within a section is optional.
The end tag is usually omitted, but in some instances the end tag may be necessary. For example, when a section is followed by an <rsect> element that is on the same level, an end tag for the section is required. Without the end tag, the <rsect> element would be considered a subsection of the section preceding it.