on an Eos it is simply an empty key. on a Gio you can reassign it. by default it's a macro key for macro 821. you can also reassign it in the shell, settings, buttons, scroll down to the Gio section. you can make it any hard- or softkey button or a macro number of your choosing.
it's only customizable on Gio and Ti though.
my personal favorite for that button is MagicSheet
So it seems in Nomad, and even using teh Wing its a dead button. Thats unfortunate. So thats two main bugs......the other is that when i go to blind, when i come back to Live, the screens dont coem back to where i was before. Annoying.
