The EW-RS910 junction box replaces the more traditional below the stem junction SM-EW90. It is mounted either in the handlebar end or somewhere built into the frame.
The junction box has two E-tube ports to connect it to the rest of the system, a charging port and a button used to check battery status and change settings on the fly.
You could replace this junction with the mountain bike System Information Display (XT SC-M800 | XTR SC-M9051), if you wanted to. This Information Display functions as a Junction A with three e-Tube ports, a display and features Bluetooth/ANT+ connectivity.
Installation and cable routing
You would usually install this junction box together with the EW-JC130 Y-connector cable. Depending on where you put it on the bike, there are several ways to do cable routing.
Junction A (EW-RS910) in handlebar
When the EW-RS910 is installed in one of the handlebar ends it is connected to both shifters using the EW-JC130 Y-connector cable. This cable connects two components to the rest of the system. In the image above the left shifter is connected to one end of the Y-connector. The EW-RS910 junction box is connected to the other end. Then, to connect everything to the rest of the system, the short cable of the Y-connector is plugged into the EW-WU111 Wireless Unit.
Note that the wireless unit is optional, using the EW-JC200 inline 2-port junction instead will work just as well.
Junction A (EW-RS910) in the frame
Some bike manufacturers have built dedicated EW-RS910 spots into their frames - usually somewhere on the downtube. These usually look nice and clean, but cable routing is slightly different from the regular handlebar bar-end junction. In this setup the EW-JC130 Y-connector directly connects to both shifters. It then connects to either an EW-WU111 wireless unit or the EW-JC200 2-port inline junction, which is connected to the EW-RS910 junction A (see upper half of image).