Watching this thread closely, I got a KC 108 a month or two ago and I'm also curious about that top right key.
Top left goes to desktop, don't recall that second one right now (not at home with my KC- file explorer or internet browser, maybe?), 3rd is calculator.
Second layers are, as mentioned, lock and sleep for the first two. Don't recall if I figured out the third one.
But that 4th key...I've pushed it, and as you can see in the picture it has a lock light on it, but I don't notice any detectable change. It doesn't seem to affect the functioning of the backlights, and I typed a bunch of stuff and didn't notice anything with it on. I have't done real exhaustive testing, but whatever it is it's not very obvious. Hope someone figures it out, I'm really curious.
PigLick