The engine rating ie 18.5K, 22K, 23.5K is the certified take off rating defined in the regulatory agency approved AFM and is usely limited to 5 min . As said before the airline choice is economical .
Your take off N1% is the speed the engine has to be rotated to achieve this thrust rating . For example CFM56-3 22K msl standard day take off N1 is about 93.7% (packs on), and this value will be increasing with an increase in temp. or pressure alt. Your Max Cont./Climb Thrust rating, for the same engine, will provide about 20.5K (isa+10°) and your max cruise rating about 19.2K (isa+10°) .
Hope that makes sense ,
I...