Shimano's E-Tube Di2 Junction Box (also known as the B junction) connects all your component and battery wires to make assembly much easier. It's a crucial item along with the five required wire sections to operate your Ultegra 6770, Ultegra 6870, or Dura-Ace 9070 Di2 shift system. The E-Tube Di2 Junction Box is available in two configurations: Internal (SM-JC41) and External (SM-JC40).