XHC WHB04 button =1/2, what is it used for?

I have installed a WHB04 on a Debian 7 LinuxCNC 2.7.8 XYZ mill. The default setup works well.

A couple of questions ==>

I am wondering what the button marked =1/2 is supposed to do? Other buttons are obvious, but not this one.

Also interested in useful commands to put on the Macro-X buttons.

It seems possible to shift the AXIS mode between Manual and MDI, (F3 -- F5) some commands do that. I cannot find the logic to make this happen. I quite often find myself in the wrong mode, trying to jog in MDI mode, trying to enter keyboard commands like G0Z10 in manual mode.

The 1/2 button is intended to find the middle of something. If you touch off to the left edge, then to the right edge, the 1/2 is supposed to take you to the middle.
I need it for other functions, so not using that.
Cheers,
/Chris