If you fly a 70 seater 1 kilometer, then you just flew 70 seat kilometers. Available seat kilometers is the summation of all the flights you have scheduled whereas Revenue Passenger Kilometers is the summation of all the seats that were actually sold on those scheduled flights. It is a stat that makes it easier to compare apples to apples. If you have 1 flight that is 10,000nm in length and a competitor has 10 flights 1,000nm in length, the 1,000nm guy is obviously going to transport more passenger. Seat miles doesn't care how many passengers transported--just how many miles flown with seats/passengers.