I once had the chance of talking to a British Airways aircrew when flying to London Gatwick from Lusaka, Zambia, about 4 years ago. I was told that flights going North fly at even-numbered flight levels, e.g. FL
32, 34, 36, 38; while flights heading South fly at odd-numbered flight levels, e.g. FL
31, 33, 35, 37. (Ot it could be the opposite, but I hope you get the idea). This rule was developed by the pilots and airlines themselves.
While I was in the flight deck, a Swiss Air flight then flew 1000ft above us -- I could see the aircraft right out the window. (It was a night flight) I first saw the Swiss Air flight about 10 minutes before it passed. Initially I thought it was a star, but the aircraft lights were too close to the horizon. The British Airways pilot also pointed-out the Swiss Air flight on the radar.
Greetings To All.