One pack IN-OP
A number of aircraft have a limit to a certain max alt if you despatch with one pack in op.
The scenario is that on leaving the ground, the remaining pack(s) have to pressurise the aircraft to the normal working diff for the particular model. Where the pack runs out of capability to do this is the the figure published as a max despatch altitude ( most probably with an approrpriate safety factor to allow for degraded performance of an individual air frame/combinations of systems).
If you are at a higher altitude and the pack fails the remaining pack(s) should be able to maintain the required output to keep the cabin where it is by maintaining the required diff pressure.
A narrow body jet from my past had the max despatch altitude of FL250.