Upon first glance, the Airbus A350-900 and the A350-1000 both look like the same aircraft scaled up or down. From the same aircraft family, with the same wing philosophy, the same cockpit, and broadly the same mission, it is not surprising that these jets look functionally the same. The jet’s long-haul efficiency is unparalleled, and the jet features modern composites. One specification, however, does effectively capture the difference between these two jets, as they each offer different maximum takeoff weights (MTOW). It is not just a larger number on a data sheet, but rather a certified limit that shapes everything from payload-range capability to runway performance. It also defines engine thrust requirements and even what an airline will pay in airport and overall navigation fees.