When Shimano Di2 E-Tube wires leave the Di2 brake/shift levers or bar end shifters, they must connect to the Di2 system via the SM-EW90 Junction Box, often referred to as Junction A or the connection box under the stem. This is where wires are plugged in to direct front and rear derailleurs and receive power from the battery. It also facilitates on-bike Di2 battery charging and system updates. The Junction Box comes in two versions: SM-EW90-A (three-port) and SM-EW90-B (five-port).