Tools > Outline Numbering defines the hierarchy of headings in a document. Headings can be numbered or not; typically the first-level headings in a book-length document are the next level of headings after the chapter titles, which may be numbered, but lower-level headings are not numbered. Some chapter title and heading styles (such as those commonly used in engineering documents) number each chapter and heading level, for example 1, 1.1, 1.2, 2, 2.1, and so on. When chapters or sections are added or deleted, the numbering is automatically changed.

Paragraph styles are the key to OOo’s outline numbering feature. The default paragraph styles assigned to outline levels are the heading styles supplied with OOo: Heading 1, Heading 2, and so on. However, you can substitute any styles you wish, including custom (user-defined) styles.

The headings defined using the outline numbering feature can be used for more than the table of contents (described in Chapter 12). For example, fields are commonly used to display headings in headers and footers of pages (see Chapter 14), and Writer can send the outline to Impress to use as the basis for a presentation (see the Impress Guide for details).