In ATR 72-500 FCOM document I found engine parameters including power settings as "Reserve Take off". What does that mean?
I've seen this mentioned in story (unfortunately in Czech) about flying in Bhutan on ATR-42 (a smaller version of the same aircraft). Basically in most situation it is recommended to use less then maximum rated power to reduce wear of the engine and that setting is marked as “take off” and the maximum rated power is only used in high density altitude (hot and high) conditions or very short runways and is labelled “reserve take off”.
Most other aircraft don't have setting labelled like this, but taking off at less than maximum rated power is common. It is called “derated” or “flex” take-off.
Engines sometimes have a "reserve power range" (or "emergency power") which is only used when the extra power is absolutely needed. This is higher than the "takeoff power" setting that is generally a 5-minute limit on certain parameters for regular takeoffs. ATR is advertising a "reserve take-off torque option" which allows higher power at takeoff. This allows using some of the extra power in difficult takeoff situations. From the materials for the ATR 72-600:
ATR has developed the Take-off at "Reserve Take-off Torque" option to improve take-off capabilities on very short runways or in case of near obstacles.
... ATR and P&WC have jointly worked to provide on request 4.5% higher thermodynamic power at take-off and max continuous ratings. This results in:
Increase of climb gradient in hot and high conditions leading to takeoff weight improvement of around 500 Kg (1,100 lb).
Example: + 500 kg from Bogota airport (8,360ft and ISA+20°C) + 1,000 ft higher one engine out net ceiling.