kijangnim,
If we were to operate our flights with maximum fuel saving in mind, we would conduct all climbs as "Cruise Climbs".
The Optimum Cruise Climb is to fly the aircraft at the Maximum Range Cruise Speed (MRC), with the thrust set at the optimum speed for Thrust Specific Fuel Consumption (TSFC).
Typically (but PLEASE check the characteristics of your own aircraft), Normal full Climb Thrust (MCL) is at an engine speed somewhat above optimum TSFC speed. Optimum TSFC is more likely to be found closer to MCR. Thus, Climb at a derated Climb Thrust is more than likely to be closer to optimum TSFC engine speed. Of course, for maximum efficiency, the speed flown should be as close as possible to MRC.
At lower altitudes this does make some sense, but, as we are within 10,000 feet or so of Optimum Cruise Level, the Rate of Climb drops to operationally "annoying" levels. Thus, as a good compromise between optimum fuel savings, and practical aircraft operations, Climb at De-Rated Thrust makes good sense (for fuel economy) at lower levels, and at full MCL at higher levels to provide a reasonable level of aircraft performance.
It's not my idea, Boeing and Airbus beat me to it.
De-Rated Climb thrust automatically reverts to full climb thrust at a pre-set altitude for later generation aircraft. For earlier generation aircraft such as the A300-B4, Airbus recommended climb at MCR until the Rate of Climb reduced to 1000 fpm, whereafter they recommended increasing to full MCL.
Nothing's new is it?
Best Regards,
Old Smokey