Typically you have either Take-off Rated Thrust (TRT) which is limited to 5 minutes. Or MCT which stands for Maximum Continuous Thrust. So you take off in TRT and then set MCT. Usually pilot sets certain engine parameters, releases brakes and co-pilot adjusts to TRT. Once aircraft gets of the ground and cleans up, pilot will set MCT.
Those values are calculated by density pressure, temperature and other atmospheric conditions.
Why? Engine wear and tear. Most engines can go up to 105%+...but they are limited by ITT or other limitation. So if you run at 700C for X, that engine will have to be replaced or refurbished. The only time you want max thrust out of your engine is if you don't want to hit terrain...given that it has a 1.0 k/d ratio.
FYI difference between MCT and TRT is small....680C and 700C for engine I'm playing around with right now.