The assumed temperature thrust reduction is limited to no more than 25% from the selected take-off rating level. This is to ensure that the FIXED derates of CLB1 and CLB2 (10% and 20%)will not be 'voided' by an assumed temp take-off derate being BELOW the armed values for derated Climb. In other words, it avoids the thrust being INCREASED when the CLB derate is activated at the thrust reduction point. This is done by automatically reducing the assumed temperature when entered, IF it would have produced that excessive derate.