When you are trying to get your arms around how Di2 wire kits work, it is best to think of the wire kits in terms of front and rear meaning, one wire kit for the front half of the bike and one wire kit for the rear half of the bike. If you could imagine looking at the profile of a bike and drawing an imaginary line right down the center, you would have a wire kit for the front half and a rear wire kit for the rear half. There is no longer right and left like standard cabled and housed mechanical shifting bikes.
This kit would then be the rear wire kit. At the center of the kit, it has a center junction box in which four cables come out. One goes to the bracket mount for the battery which mounts underneath the water bottle cage. One goes to the from deralllieur, while the other one goes to the rear derailliuer. the last wire connects to the front wire kit.
There are three sizes to choose from short, medium and long. While most frames take a size medium, very small or very large frames may require purchasing the short or long size rear wire kits.
Kit includes one full rear wire kit, one TW-EW01 tool (used to plug and unplug wires without damage) and one package of black cable covers which are an adhesive tape made specifically to route and hide the cables.