I know this started as an Airbus thread, but this is an interesting discussion for Boeing folks, too.
From
Jet Transport Performance Methods, Blake, et al 2009.
Thrust Ratings
So what is a thrust rating? Simply put, a thrust rating is a maximum level of engine thrust that is permitted for a specific phase of flight. For example, takeoff has its own specific thrust level, called Maximum Takeoff Thrust. That level of thrust can not be used at any other time during the flight. Also, as you’ll see, it can be used only for a specified number of minutes when taking off.
Normal climb to altitude has a different thrust level called Maximum Climb Thrust, or MClT. Similarly, cruise has its own rating, called Maximum Cruise Thrust, or MCrT.
For emergency purposes such as driftdown following an engine failure in cruise, there is a specific thrust rating called Maximum Continuous Thrust, or MCT.
A fifth rating exists, called Go-around Thrust, sometimes also referred to as Maximum Inflight Takeoff Thrust. This is a special rating used only at lower speeds and altitudes such as during a missed approach when maximum possible performance may be required.
The Five Thrust Ratings
At the beginning of the chapter, we said that there are five separate thrust ratings, each applicable to a particular phase of flight. Three of these are called certified thrust ratings, meaning that they are the basis of airplane performance
data which is governed by aviation regulations such as the United States Federal Aviation Regulations and thus are published in the Airplane Flight Manual (AFM) and have the force of law. Compliance with these certified thrust ratings is mandatory.
maximum continuous thrust
This is the third of the “certified” thrust levels.
Maximum Continuous Thrust, or MCT, is a special thrust rating that is only usable in the event of some emergency situation. It may not be used in normal operation.2 MCT is the greatest amount of thrust that can be used in flight, with the exception of takeoff and landing.
2. For some engines, the maximum continuous thrust rating is the same as the maximum climb thrust rating; in those cases, this statement does not apply.
In the 737NG, MCT is the same as CLB. Following note 2, it
may be used in "normal operation."