Look at the photo of the receiver... the sixth slot is labeled 6/B.
This is the 6th channel, with the receiver battery and the servo for that channel plugged into a y-harness, which plugs into that slot.
BEC is a feature of some ESCs which power the receiver and servos as well as the electric motor.
The BEC will keep the receiver and servos powered when the ESC shuts off the motor at some low voltage level.
ESCs will have limits as to how many servos they are capable of handling.