The Shimano Di2 SM-JC40 is designed to connect SD50 E-tube wires along four points; front derailleur, rear derailleur, cockpit junction and battery mount.
The B-Box is used to split the main wire from Junction A to the front and rear derailleurs. It can also be used to connect a seatpost mounted battery to the system.
Specifically designed for externally mounted systems and mounts underneath bottom bracket shell - for External systems use SM-JC41
