The intent of this Success Criterion is to provide headings for sections of a Web page, when the page is organized into sections. For instance, long documents are often divided into a variety of chapters, chapters have subtopics and subtopics are divided into various sections, sections into paragraphs, etc. When such sections exist, they need to have headings that introduce them. This clearly indicates the organization of the content, facilitates navigation within the content, and provides mental "handles" that aid in comprehension of the content. Other page elements may complement headings to improve presentation (e.g., horizontal rules and boxes), but visual presentation is not sufficient to identify document sections.
This provision is included at Level AAA because it cannot be applied to all types of content and it may not always be possible to insert headings. For example, when posting a pre-existing document to the Web, headings that an author did not include in the original document cannot be inserted. Or, a long letter would often cover different topics, but putting headings into a letter would be very strange. However, if a document can be broken up into sections with headings, it facilitates both understanding and navigation.