I was originally assigned 61J. I used on-line check-in and reassigned myself to 62A. You can use on-line check-in 24 hours in advance of your first flight. If your are any level in BAEC. You can also register as a non-Exec but you can only check-in 12 hours in advance. The reason I stressed first is that if you are connecting you can gain an advantage over people commencing theur journey at the main point of depature. For example, my first flight left EDI at 15:15 on 24th Dec. My flight to Joburg departed LHR at 20:40. I was able to check-in for both flights at 15:15 on 23rd Dec!