As an improvement over the Dura-Ace Di2 setup, the Ultegra Di2 group uses a junction box to simplify the wiring, giving you a cleaner look with easier setup. The junction box mounts under your bottom bracket shell (for external wiring) and connects the wires coming from the derailleurs, levers and battery.
Junction box mounts under your bottom bracket shell
Provides a secure, waterproof connection for your wiring
Compatible with externally-wired Ultegra Di2 setups