From Manufacturer
The Main Control Board for 350 QX2/QX3 Quadcopter from BLADE is the central hub to which all of the aircraft's major electronic components connect. It is also the brains of the aircraft, keeping it in the air and translating pilot stick input into motor response. Hardware that connects to the Main Control Board includes the receiver, ESC, compass, GSP receiver, battery, and more.
Note: This is for the 350 QX2 and 350 QX3 models only for the original 350 QX use BLH7801A instead. Also note, connecting certain components to the Main Control Board may require splicing wires, heat shrinking, or soldering.