Ratio of air to fuel. Think of it as a weak mixture scenaro. The FCU will adjust fuel flow for normal operations, anywhere near limits the FCU will not compensate for those atmospheric variations. Add this ti compressor and power turbine blade performance criteria and there you have it. You'll never get an engine that is all things to all men. They are designed within a tight set of requirements and this will post a set of limitations.
Or were you seeking a more scientific reply?