Getting a little esoteric here and my aero eng quals are distinctly 'historic'. One for the theorists, I feel. For we simple 'drivers, airframe', suffice it to say the donk is not so good at 2000'AMSL as at 0.
It doesn't really matter to us, since clever folk like JT give us numbers we look at. When/if Tapshi returns, he/she can explain what system the airline uses to establish MTOW.