A Topic object represents a concise chunk of text, usually no more than a few paragraphs. In traditional content, this would be the text under a specific heading or subheading in the document hierarchy.
Topic objects make up the vast majority of all objects in a book, and as such contain many properties defining how the topic is represented in both the printed and online outputs. Topics can mark the beginning of a new chapter or section, and automatically create a mini table of contents. The heading of the topic is automatically formatted based on its level in the hierarchy of the book.
Videos: Creating your Content | Editing Controls | Editing Topic Headings