I usually see "flat rating" associated with turboprops.
Turbofan engines are often derated to extend engine life and increase allowable tolerances before hot section overhaul is required. Our 744s have B1F and B5F versions. Most of the B5Fs have been derated to B1F to decrease maintenance costs.
The CF6 engine will maintain rated or selected thrust after TO as long as possible (or until a CLIMB thrust is selected). The EEC does it by adjusting N1. Since a lower CLIMB thrust is usually used, "flat rating" doesn't have a lot of practical application in airline practice. The Air Force may treat the engines differently...