Read an availability screen, understand booking classes, and sell segments correctly.
1 MS 123 J4 C6 Y9 B9 M9 K2 CAI LHR 0810 1215 738 — line number, carrier, flight number, then the booking classes with the number of seats the airline is willing to sell, then the city pair, departure/arrival times and aircraft type. A class showing 0 or a letter with no digit is closed.
Y, B, M, K, H, Q, T, V are all economy — they differ in fare level and restrictions, not in the seat. J and C are business. Selling the wrong class is the most common trainee mistake: the passenger flies the same cabin but pays a fare with different change and refund rules.
Sell by referring to the line number that is currently on your screen. If the availability display is gone, the sell entry fails — display availability again first.
SS1Y1Sell 1 seat in Y from line 101Y1Sell 1 seat in Y from line 1N1Y1Sell 1 seat in Y from line 1