The Di2 EW-SD50 Wires are the "E-Tube cables" for Shimano's Ultegra 6770 and Dura-Ace 9070 groupsets. They're used to connect each of the electronic components within either system.
You'll need four of these to connect the Front Wire/Junction (A) to the Rear Junction Box (B), Battery, Front and Rear Derailleurs (all sold separately). Dura-Ace requires two additional wires because the shifter control wires are not integrated into Front Junction Box (A) as they are with Ultegra.
These are sold individually. To recap, four wires are required per bike for Ultegra, and six wires are required per bike for Dura-Ace. Choose from various lengths for a perfect fit*. Compatible with internally or externally routed systems. The SM-GM01 and SM-GM02 Rubber Grommets are recommended for frames with internal cable routing. For external routing, add the SM-EWC2 Wire Covers for an additional aesthetic touch.
*Note: Additional wire slack can be taken up at the Rear Junction (B). When in doubt, it's best to order a slightly longer wire.