That the seats are available for sale is critical. An aircraft that had 300 seats but which was limited (for regulatory or technical reasons) to selling only 250 of them on a particular flight of 1,000 miles would generate 250,000 ASMs on that flight, not 300,000. Similarly, if passenger seats on a particular flight are dedicated to crew rest purposes, such seats would not be included in the calculation of ASMs for that flight.