After Jan told that derated and reduced thrust is not the same, I had to find something out about this. In addition to @mmerelles nice explanation I found a short version from a other forum.
Assumed temp (reduced thrust) – take-off with less than 100% thrust but you can push the thrust up to 100% if needed.
Derated – take-off with less than 100% and that’s all you have (can not or should not push the thrust up.)
Or maybe this one is better.
When take-off power is based on reduced power, whatever the method for establishing the reduced power, all limiting speeds are based on full rated power. Vmcg and Vmca are the most limiting speeds.Fixed derate, however, is like taking one engine off the plane and installing one of greater or lesser thrust rating. Vmcg and Vmca are based on the rated power being used, the full thrust or the fixed de-rate.