If you are talking about the JAA exams Barnaby's explanation should suffice. Does anyone know why the engine is flat rated at temperatures below ISA+14 or ISA+15? I have never been able to find a complete explanation. The best I have so far is this:
Engines can be limited by their internal temperatures, usually TGT, or by an RPM limit. If the outside air temperature is high, say 45º, then the first limit you hit as you advance the throttles is the TGT limit. If the OAT is lower, perhaps 35º, you can wind more RPM on before you hit the TGT limit. In this range of temperatures thrust increases as temperature reduces.
There comes a point, though, as temperature continues to reduce and RPM is increased where you hit the RPM limit before the TGT limit. From this point on the maximum thrust you can wind on stays constant and RPM limited.
Harrier pilots have an engine limit they call NF root theta. As explained to me, NF is the speed of the fan and 'root theta' is the square root of the temperature in degrees absolute, a Mach number limit. This limit kicks in at low temperatures when the speed of the fan is so high it creates shock waves in the engine. Could this be what causes the RPM limit in other high bypass ratio fans?