Topics and subtopics form a hierarchy below the home topic. Typically, the first level of topics following the hometopic are divided into chapters, using the <chapter> element. Within a chapter, topics are organized into sections. Subtopics of an <s1> section are entered with <s2>, subtopics of <s2> entered as <s3>, and so on.
Either element, chapter or section, can follow the home topic. There is no visible difference to the user if you start your hierarchy with <chapter> or <s1>. Figure 2-1 shows a simple hierarchy that includes three chapters. Each chapter contains several first-level sections. The third chapter adds two second-level sections.