Because standard RJ-11/12 Sensors put their blocked/clear output on Pin 4; this type of Sensor can be plugged into the Downstream Control Port and provide the Pin 4 Lane Full signal.
When PEx is plugged into the Right Control Port and its output is ON, Zone 1 will see a Lane Full condition and stop any item that arrives at PE1. If PEx output is OFF, Zone 1 will continue to run and convey item downstream