Castle Creation Pro BEC "Battery Eliminator Circuit" is a small device that removes the need for a reciever and servo battery pack. It will take the high voltage from the multi-cell main battery pack and regulate the output for the reciever and servos. This is great when you need a large amount of power for hi-performance servos or use packs with 3s or more (more than 7.4 volts) for your main battery packs. Most ESC's with internal linear BEC's are not designed for this.
The Castle 20A PRO BEC V2 is intended for use in larger RC cars, electric aircraft, and helis that have high power demands for their radio/servo systems. It is ideal for most electric helis 500-size and larger. It will also work well in larger electric powered RC aircraft and RC crawlers and Monster Trucks that have high-powered servos.