It doesn't matter what machine your talking about. The terms OPEN LOOP and CLOSED LOOP denote the same condition on all machines.
The terms refer to motor feedback and control.
When you have open loop control there is no feedback.
When you have closed loop control, there IS feedback.
For example. Put a blindfold on and go to your Mill. Using the hand wheel, move the X Axis. (this is your output) I'm sure you can move it, but because you have a blindfold on you wont know how much you moved it because you cant see it.. (no feedback and thus an OPEN LOOP control scheme)
Take off the blindfold and now you can use your eyes to tell you how far the machine has moved. Your eyes are the feedback loop so in essence, your hand moved the wheel and then the machine moved, and then your eyes told your brain how far it moved. This is called CLOSED LOOP because you have feedback telling you what your input did.
In a CLOSED LOOP control scheme, the feedback happens thousands of times per second which allows your control to update and change the input thousands of times per second which is what allows presicion control over a motor.
I suck at explaining things eh?
Hope this helps