In fact, ATM is only available for the takeoff, while fixed derate can be chosen for both takeoff and climb. In the former, the engine power is reduced to the value at which one may start at maximum with the planned TOW, but keeps the reserve up to the maximum possible thrust value. For a Fixed Derate it is assumed that you have a "smaller engine version". The maximum thrust value is then really limited to this lower value without reserves, while in the case of the ATM method, the thrust levers can be moved completely forward and can use the available reserve. It can even be combined for the takeoff both methods, but depending on the airline handled differently or allowed. Similarly, the use of the ATM is usually not allowed if the Rwy is contaminated or wet.