What is the difference between the 2 Wire and 4 wire smoke detector

What is the difference between the 2 Wire and 4 wire smoke detector


2 wire smoke detectors connect with only 2 wires back to the panel, and can only be connected to the 'smoke zone' on the panel.  Their is typically one zone set aside for this on each brand.

4 wire smoke detectors use 2 wires for power, 2 wires for signal, and can go on any zone as long as you program the zone appropriately, which also means you can know WHICH smoke picked up smoke. But they won't all go off unless you interconnect them