You have a couple of descriptions mixed in there.
Steppers generally are never closed loop as there is no feedback device, servo's operate with a feedback device of some kind and close the loop via a PID method.
With a system such as Mach where the control is through the parallel port, the loop is closed back to the servo drive, the loop is not closed back to the CNC controller (Mach).
The controller sends a command and assumes that the position will be reached.
With the addition of a motion card such as Galil, the control can close the loop back to the controller.
This has the advantage of features such as electronic cam and electronic gearing, as well as true synchronized threading and helical milling etc.
If you are also talking of DRO or scales on the final positioning, then dual feedback is required, an encoder on the motor and a scale on the final positioning device, the PID loop is then split between the two feedback devices.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.