Shimano's Dura-Ace Di2 takes shifting to the next level. The Rear Wiring Kit connects to both the left and right shifters and Junction Box A (which displays battery power and allows manual adjusting). It also includes a battery mount, cable covers to route and hide the cables and a wire plug/unplug tool. The External Kit is meant to be run on the outside of the frame.
Measure your frame according to the diagram and calculate X. Choose the wiring kit that’s the same length or longer than X.