In the 2023 HD it is used for light bar and heater fan. In the 2018 it is used for light bar, sound bar, and auxiliary lighting, and sometime later the auxiliary lighting was replaced with heater fan. In my 2023 HD I used it for forward and side roof mounted light bars on one switched circuit and dome/cargo lighting on the other switched circuit. It contains a common ground and two hot circuits controlled by two switch harness connectors behind two of the switch blankoff plates. The connector is still pinned out exactly as shown in the 2018 wiring schematics. Just replace the two blankoff plates with two switches and you have two ready made switched circuits. Fuses and C/Bs already installed in my 2023 HD.