LibreOffice Navigator serves as an outlining tool and a working table of contents. However, the default display is limited only to Heading paragraph styles, which means the outline is limited to sections rather than paragraphs. Fortunately, you can improve the default using outline levels.
Most users assume that Tools > Outline numbering is just a means of creating outlines using Heading paragraph styles. In fact, outline levels are far more important than they appear, because they determine which paragraph styles are used to automate features such as cross-references and tables of contents — and, more importantly, the paragraph styles that display in the Navigator.
Outline levels are associated with Heading styles, so that the first outline level is mapped to the Heading 1 style, and so on. You cannot change these mappings, but you can map additional styles to an outline level. All you need to do is go to a paragraph style’s Outline & Numbering tab, and select an outline level from the drop-down list in the Outline level field. After you click the OK button, the style will display wherever outline styles are used — including the Navigator.
However, to avoid confusion, you should add any additional styles to an outline level that is not used in your document. You should hardly notice this limitation, because LibreOffice provides ten Heading paragraph styles, which is far more than any document can actually use. Use too many Heading styles, and both you and the reader are probably unable to distinguish one from another, no matter how ingenious you are at trying to differentiate each of them by indentation, font selection, font style, font size, color, and line spacing. Rather than attempt such a hopeless effort, restrict yourself to three or four instead. Your document can only benefit.
More to the point, only using a few Headings leaves you with six or seven you can assign to non-Heading styles. For example, if your document uses three Heading styles, you can use the fourth outline level for Text Body and Text Body Indent, the paragraph styles that most of your document is likely to use unless you have chosen to use a style of your own instead. In many documents, you probably have no need to associate any other style with an outline level, but, if you do, you are still left with several outline levels to play with.
Of course, Heading styles do have one advantage over Text Body and Text Body Indent: they are generally short enough to fit into the default size of the Navigator. By contrast, a Text Body paragraph is frequently several lines long. Unfortunately, the Navigator is unable to wrap text, but you can undock the Navigator from the sidebar, then increase the width of its window by dragging on its edges so that more of a Text Body paragraph is visible. The increased width should display enough for you to identify Text Body paragraphs within the Navigator — and to make the Navigator even more useful than it is by default.
(Based on my forthcoming book, Designing with LibreOffice)