Lots of miss information here (mixed in with some correct information).
Most FADEC engines have multiple thrust ratings available - the CFM on the NG is no different. A 'rating plug' or 'data entry plug' plugs into the FADEC to tell it which rating to select. The rating plug is typically considered part of the engine (the rating is listed on the engine data plate - an engine company service bulletin is required to change it, which changes the rating plug and data plate) and connected to the engine by a lanyard (e.g. if a FADEC is replaced due to a fault, the rating plug remains with the engine).
The engine is flat rated for temperature up to corner point (as noted, typically ISA plus 15 - 30 deg C at sea level but decreases with altitude). However it is
not flat rated for altitude - max thrust decrease with altitude.
For EPR engines, max rating EPR is basically constant at a given altitude up to corner point temperature, however N1 engines have a constant
corrected N1 below cornerpoint - but we set and display
physical N1 (physical N1 is corrected N1 * the square root of Theta). So, below corner point, physical N1 increases with temperature to maintain the same corrected N1 and thrust.
On Boeing installations, with the FADEC in "Normal" mode, Max rated takeoff thrust always occurs at the same throttle position - as does max climb and max cruise. Max Takeoff throttle position occurs a bit short of the max forward stop, push a bit beyond that and you get what we call 'headroom' - slightly more N1 to account for small sensor errors between engines, etc. (headroom is typically between 0.5 and 1.0% N1, depending on the conditions). The last degree or so of throttle to the forward stop will NOT increase thrust - it's basically a flat to insure that a mild throttle miss-rig won't prevent obtaining max rated thrust. Aside from the noted headroom, overboost is not possible in Normal mode (baring a major failure).
Now, with all that background to put you to sleep
, here is the response to the original post:
The FADEC monitors ambient conditions - inlet total temperature, ambient pressure, and total pressure (i.e. temperature, altitude, and airspeed). The FADEC then calculates the correct max Takeoff, Climb, and Cruise values for the selected engine rating. It will then apply any 'debits' to those ratings to account for aircraft bleed (as communicated from the aircraft). Those values are then compared to the throttle position to determine the "Command" N1 value and the FADEC goes about controlling the engine to obtain the Commanded N1 for that throttle position.
'Alternate' mode is completely different - it's an unrated mode and significant overboost is possible.
Clear?