Some items are just simple and straightforward. If you are setting up or modifying your Shimano Di2 e-Tube wiring system and require a two-ended female-female adapter, this EW-JC200 E-Tube Port is the item you need.

So essentially this part allows you to join two male e-Tube wires. At just 1 gram in weight, it won't slow you down, and the 31mm length will hide away nicely.