r</code> property is the number (or letter) defined in the <code>number</code> option of the element in <code>config.yml</code> file. Chapters and appendices must define six different labels, each corresponding to one of the six headings levels (<code>&lt;h1&gt;</code>, <code>&lt;h2&gt;</code>, <code>&lt;h3&gt;</code>, <code>&lt;h4&gt;</code>, <code>&lt;h5&gt;</code> and <code>&lt;h6&gt;</code>). In the following example, appendices only include a label in its title, thus leaving empty the last five levels: