The E-Tube Di2 Junction Box is what is referred to as the B Junction. This is the point in the wiring system where all the component pieces converge. It's a crucial item along with the five required wire sections to operate your Ultegra 6770 or Dura-Ace 9070 Di2 drivetrain.
The E-Tube Di2 Junction Box is available in two configurations -- Internal and External. The Internal style is designed to be located inside the bike frame (usually inside the down tube, tucked in from a hollow bottom bracket shell). The External version is designed to be fixed on the outside of the bottom bracket shell, usually fastened to the cable guide.