As a newer user of BIAB (6 years or so) I can tell you these markers are essentially useless. Completely non-intuitive so I just ignore them (like lots of obscure or poorly documented BIAB features unfortunately.) I suspect I am not alone among non-BIAB-old-timers. I just stumble around in the program until I get some RealTracks I like for my song, export them as WAVs and exit the program as quickly as possible!
This wishlist request has been here as long as I have and it would be so simple to add as it is nothing but a list of definitions. Why they have not done it is baffling!