Electric Wire / E-Tube cable
The next-generation Shimano EW-SD300 electric wire is used to connect the individual parts of the EP8 E-MTB and future Di2 series to each other. They're available in a lot of different sizes:
These wires are smaller than the older EW-SD50. The wire diameter is about 2.1mm and the plug itself is 3.2mm.
You cannot use the TL-EW02 tool to connect these wires, it is too big. The correct tool is the TL-EW300.