This was a rather cumbersome feature that existed in the past to address a specific use case that can now be done more effectively in other ways. The feature was removed in Tiki 15, and this page is linked from the release notes for that version.
The idea was that you could create a category field in a user tracker, the parent category under which there were categories which had category names that matched certain group names. Then when a user was added to (or removed from) a particular group the tracker item would be categorized or uncategorized with that category respective. Similarly if those categories changed, the user would be added to (or removed from) the group respectively.
This was originally done to be able to display info from a user tracker filtered for whether a use was in a certain group or not. It also provided a way for users to edit their membership in certain groups through editing the User Tracker. Since the feature was added, certain things have happened:
1) The User Groups field actually is a simpler way to achieve at least first part of the use case.
2) Users can now be indexed in unified search and the group info too, so you can show the information filtering from there using LIST or CUSTOMSEARCH as needed.
3) Wikiplugin_subscribegroups.php has been the better way to allow users to edit their group membership
4) It is discovered that it is too easy to accidentally change group membership when editing user tracker item as it can be unclear that that that will happen.
5) When a user tracker item is recreated for any reason, it causes all the group memberships to be wiped out inadvertently.