A UBEC (universal battery eliminator circuit) is purely a device that converts high-voltage (such as 25V from a 6S LiPo) to lower voltages to allow the full UAS to be powered from one primary battery source, The UBEC is a Switch-mode DC regulator separated from ESC (ESC—) Electronic Speed Controller for brushless/brushed motor), it will take high-voltage (5.5V to 26V) power from your battery pack and convert it to a consistent safe voltage for your receiver, gyro and servo.

For traditional speed controller with a build-in BEC, it will very likely have only a limited ability to supply power to your receiver and servo without overheating. if you are using a high-voltage battery pack or have heavy servo load, you should consult the ESC specification to determine what the started recommendations or limitations are. in general if you are using a 4 cell lithium battery pack or more than a 12 cell Nickel based battery pack you should consider using an UBEC because  in such a case the build-in BEC of the ESC can support only 2 servos, which means it is not suitable for RC helicopter and big aircraft.