TRACKBAR (located just below PLAYBAR) is a plain black horizontal bar by default. If you have activated TRACKER (see below), TRACKBAR will show what HH'MM (hour:minute)/frequency pairs you have already listened to, and how much time you have spent on each HH'MM/frequency pair.
TRACKBAR is fully aligned with PLAYBAR's playback time: each minute on PLAYBAR has a corresponding minute area below it, and you can see thickening vertical bars based on how many seconds you have spent on each particular minute on PLAYBAR.
The TRACKBAR area is also used to display the top-of-the-hour (TOH) and bottom-of-the-hour (BOT) markers for PLAYBAR if the file being played back contains an exact TOH/BOT period (see PLAYBAR for more details).
TRACKER can be enabled/disabled via SETTINGS > SYSTEM > TRACKER > YES | NO. If enabled, JAGUAR collects the listening activity tracking data, i.e. data on how much you have spent on each frequency in any specific HH'MM. This data is saved to the YAS.LOG file.
TRACKER does not have anything common with YAS logs but because both are internal daily data collectors, both results are saved to the same file. No need to maintain two separate files in the background.