The short version is; how do you associate (i.e. link, so their state is mirrored) two endpoints on a single device (i.e. on a 4-channel switch, link button 1 and 2)?
The Z-Wave JS UI is a little too clever, as when you open a device and go to Groups and Add, the UI shows you a list of target devices but it cleverly filters out itself (i.e. the device you are associating from), because I guess the assumption is you would always be selecting another device to associate, because the normal use-case is make a switch over there control this device over here (i.e. there would be two devices, a source and a destination).
But in my case, I have a 4-channel MCO MH-S314 switch and I want button 3 to turn on several other devices AND button 1 of that same panel. I can get it to turn on all the other devices just fine, but the UI won’t let me select button 1 of the same device!
I could create an automation to do it in HA, but I am moving from that to use Z-Wave associations because they are much quicker and more reliable than it having to bounce the request across several nodes to HA and back again when the association is near instant as the signal goes direct to the nearby devices.