The "GND" terminal on J11 is isolated from the computer interface by the PMDX-126, so the PMDX-126 and computer are protected. This signal is intended to be safe to use with the safety ground and/or chassis of a CNC machine.
Other parts of your machine wiring that also connect to the "GND" terminal of J11, J12, and J13 of the PMDX-126 will all be connected to the same circuit. Normally the items connected to these "GND" terminals are designed to also be safe to connect to the safety ground and or chassis of the machine, but I wanted to mention that for completeness of this response.
If you are using a simple isolated probe plate connected to the "E" input on the PMDX-126, your cutter/probe is expected to complete the circuit back to the "GND" terminal. While many machines will work by using the chassis to complete the circuit, I recommend using an "alligator" clip on the cutter/probe with a wired connection back to one of the "GND" terminals for more reliable operation.
