Every Document has a Document Navigation. By default, the Navigation will be the Navigation View of the Document Content. (Previously known as the outline).
The Document may have a navigation
field defined, which will override the Content that is used to generate the Navigation. This allows authors to control how readers will go deeper into the content, and will probably contain Cards and Queries that expose Sub-Documents.
A Document Navigation can include an embed of the document itself, which allows an "outline" to appear in the Navigation UI, helping the reader jump around headings inside the document.