The Shimano SM-JC4041 Rear Junction connects all of the Ultegra Di2 electronic components to the battery unit. It also serves as the interface between the Front WireJunction which transmits data from the controls and the Di2 Derailleurs. This piece is also called Junction B and is available in two versions SM-JC40 for externally routed wires which mounts under the bottom bracket shell and SM-JC41 for internally routed wires which mounts inside the down tube just above the bottom bracket shell. Neither the SM-JC40 or SM-JC41 Rear Junction is compatible with Dura-Ace Di2 components.