What is a design mission and what parameters define it?

A design mission specifies the primary operational capability the aircraft must achieve. Parameters include: Range - Distance to be flown with specified payload; Payload - Passengers and/or cargo weight; Cruise speed/Mach - Design cruise condition; Cruise altitude - Optimal or specified altitude; Takeoff/landing field length - Runway requirements; Reserve fuel - Regulations specify minimums (NBAA IFR, FAR, etc.); and Diversions - Alternate airport requirements. Mission profile includes: Taxi, takeoff, climb, cruise, descent, approach/land, reserves. Design mission drives sizing: Wing area for takeoff and cruise, Engine thrust for climb and field length, and Fuel capacity for range with reserves. Airlines/operators specify design missions based on route structure. Variants (range, capacity) share common design but optimize for different missions.