I personally do not even know all the priority codes. However;

2A-Pax with biz class same day rez
2B-Pax with biz class with rez on different day
2C-Pax with rez for later flt, going earlier
2D-Pax who missed flt
4A through N-non revs
5A through D-non revs buddy pass, other airlines

These are then sorted by fare class and time added to list.

My guess is that the agent who listed you did not know what they were doing. You should have been before any coach pax. Also, it is a common practice to oversell coach. If that does end up overselling the flight, it will show no availability in biz even if only a few people are manifested.