Are you in a film project by chance when you see this?
The letters represent the pulldown cadence of that frame in context of a 24 frame edit with a 30 frame master:
|A1 A2|B1 B2|B3 C1|C2 D1|D2 D3|
In SD projects, it is possible to insert edit into a master and maintain 2:3 cadence which is preferable for 30i masters from 24 frame edits. I don’t believe Avid carried that feature over to HD (unfortunately). The information is also available in EDL’s for downstream processes to not only know frame, but cadence type.
Michael