What is meant by derating in this context?
The maximum current rating of the drive is (say) 12A RMS on the nameplate.

The drive can operate within that limit over the range 0 to 40°C.

If you must operate at a temperature higher than 40°C you must derate the nameplate rating by 4% per K, up to a maximum of 55°C (15K difference) at which temperature the drive is only capable of 40% of the nameplate rating (100% - (4% * 15K)), or 4.8A RMS in my example.

The drive should not be operated at all below 0°C or above 55°C.