I am not a professional machinist so please forgive my ignorance.

As I understand it, "referencing" an axis means to "home" the axis.  I understand that this moves the axis to its home position and sets the DRO to 0.  This indicates to the operator that the machine knows its position with respect to that home position.  I do this every time I turn on the machine. 

The "Using Mach3 Mill" document says that "The De-Ref All button does not move the axes but stops them being in the referenced state."

When and why would I push that button?

Thanks...

Its hard to think of many situations and I suppose 99.999% of the time you would never need to do it.
I do use it in my RefAll button on a couple of machines as I do the referencing externally in my servo drives. I look at the state of the Home LEDs and thus I De-ref to get them to the state I wish.
Hood

that clears it up, thanks Hood.