This area of the screen allows you to see where you are in the Chapter/Page structure.
There are four chapters:

NAV - Navigation pages including the moving map and CDI screens
WPT - Waypoints information
AUX - Auxilliary functions
NRST - Nearest (airport, navaid, intersection)

The boxes to the right of the Chapter Name indicate the number of pages in that chapter. The
white box indicates the page currently displayed.