From boeing and Airbus :
Maximum Altitude
Maximum altitude is the highest altitude at which an airplane can be operated. In today’s modern airplanes it is determined by three basic charac- teristics which are unique to each airplane model. It is the lowest of:
• Maximumcertifiedaltitude(structural)thatisde- termined during certification and is usually set by the pressurization load limits on the fuselage.
• Thrust Limited Altitude – the altitude at which sufficient thrust is available to provide a specific minimum rate of climb.
• Buffet or Maneuver limited altitude – the altitude at which a specific maneuver margin exists prior to buffet onset.
Although each of these limits is checked by modern flight management computers the available thrust may limit the ability to accomplish anything other than relatively minor maneuvering.
The danger in operating near these ceilings is the potential for the speed and angle of attack to change due to turbulence or environmental factors that could lead to a slowdown or stall and subsequent high altitude upset.