End-of-Line Resistor is a resistor placed in an alarm zone circuit, ideally at the last device on the loop, allowing the control panel to “supervise” the circuit (make sure that everything in that circuit is intact), and allow functions like mixing normally open and normally closed devices in the same circuit/loop. Many systems provide an option for turning off the requirement for these resistors, but they are usually still required for zones set as fire/smoke loops.