The navigation pane is to the left of the content pane & has several different tabs in it. These include Contents, Index, Search, Favourites, History (empty), Author (hinted at in htmlhelp.h, but currently can't seem to get it) & custom tabs.
The Contents tab is a tree structure of titles. Each title may contain other titles and or link to a topic file. It is used by authors to list all the contents of a help file.
The Index tab is also a tree structure, but it usually is a list of words linked to topics. It is used by authors to list all concepts in the help file & is usually linked to context sensitivity.
The search tab allows users to search topics for keywords. The advanced mode allows a user to search using boolean, wildcard, and nested expressions. A user can also limit the search to previous results, match similar words, or search topic titles only.
This is a list of bookmarks the user can add topics to & remove topics from. It is stored in the hh.dat file.
These are currently used in MS Office for the Answer Wizard and "enhanced" Index tabs, but they don't seem to work in HH & seem to require an different program provided by MS Office. No doubt MS has actually completed this technology but is withholding it from use by the general public, who rely on HH. Why they would do this is left as something for the reader to think up. Strangely enough it is possible to put custom tabs into the CHM, but the viewer architecture must not yet be complete since they are not displayed.