Max altitude
I'm wondering what can legally limit max altitude.
My 172's documents says that the ceiling is 14000 and max operating altitude is 17500.
I understand that climing above 14000 might take a long time or might not be feasabile at all, and that climing above 17500 is forbidden.
Why is it forbidden to fly above 17500? What could be the limiting factor?
My question is rather for the DA42 the ceiling of which and max operating altitude are both 18000ft. What certification critierion is not met if the plane is flown above 18000ft?