What?
Just what it says: the ability to mute the guitar signal for the headphones out, so you can just hear the aux signal alone on your headphone or IEM's.
Why?
I'm using a wired in-ear monitoring system and I (or the monitor engineer) wan't to control the entire monitor mix from one device. Because simpler = better.
Now I'm bringing a separate headphone amp next to my Kemper and I need a power adapter + extra power socket on stage. I wan't to get rid of this because simpler = better.
Kemper already made setting up my rig a lot simpler (just the Kemper and remote, no micing needed it is the dream!!)
But If I just could mute the guitar on the headphones out from the Kemper, I can feed my in-ear monitors from the Kemper headphone out and I don't need my separate headphone amp and power adapter on stage It would make it even better. 
Can't you just mute the guitar signal in your monitor signal?
Yes (and sometimes no!), but when you do it, you need to work with the Kemper and the monitor mixer. That is two devices you need for controlling your entire monitor mix, and that is a lot more complex if you do it your self, but almost impossible if a monitor engineer wants to make a perfect mix for you.
Didn't you posted something like this already?
Yes, but this feature request is a more simple request than the request I posted before:
Headphone mixer as an extra Output page
That request introduces more flexibility for the user, but is harder to implement I think.
So therefor I'm posting this much easier to implement (i think) feature request, and hopefully I can ditch my headphone amp soon and I'll be able to setup my rig even faster on stage as before!
