@djjd62,
It is about the New Topics list. If you have your preferences set to No, nothing changes, everything is listed chronologically according to last post date and time.
If you have your preferences set to Yes, the New Topics list is sorted with unread topics at the top and read topics at the bottom. For example, topics 1,2,3,4 and 5 are at the top of the list because you haven't clicked on them yet. You click on topic 3 and then navigate back to the New Topics list to find that topic 3 is now listed after topic 5 with the other topics you have read. The topics 1,2,4 and 5 are still at the top because you haven't clicked on them yet.