If the "Hide empty subtracks" checkbox is clicked on the User Interface on the Track Configuration Page for a composite, this feature automatically hides subtracks with no annotations in the current viewing window, maximizing space while alerting you to relevant data. The hidden empty tracks can dynamically reappear as you move to new regions where the subtracks have data to display (if they are not empty).
In this session of Problematic Regions track with the Hide empty subtracks feature activated when you zoom out, you will see new tracks subtracks suddenly appear that were previously hidden when zoomed-in:
In essence, all of the Problematic Regions subtracks are selected to display at all times, but only show up when the subtracks have data to see, explaining the meaning of hide empty. This is also a benefit as you can have this specific composite always on while doing work, and the Problematic Regions subtracks will suddenly appear if there is a reason to have a suspicion the area could lead to problems in downstream analysis while remaining hidden if there are no concerning annotations.
The hideEmptySubtracks setting comes with some other supporting settings as well. Further coming documentation to aid building indexes to speed the display when high numbers of subtracks are selected to use this feature are coming in the future.