How does the wiring work?

The MRF-300 has two cables and the AMC has appears to require three?

The manual gives the following "Limited" explanation:-



Wiring of the twin shielded cable used to
connect the remote I/R receiver to these
inputs is as follows:- The shield of the cable
should be connected to the terminal marked
"GND", the red wire connected to the
terminal marked "+12V" and the black wire
connected to the terminal marked "I/P".



I spoke to the AMC distributor here in Australia and he mentioned that the MRF would need to support a "Common Ground" to work. What ever that means?!!