Well, the end of line resistor is used to complete a circuit. To fully understand what it's for, however, we need to look at the fire alarm system itself.
Fire Alarm System
The fire alarm system isn't located just at the control panel, the fire alarm system is all over the building; the control panel just controls rest of the system.
The fire alarm system is a means of detecting a fire (smoke detector, pull station, waterflow switch, heat detector, etc.) and a means of warning people about the fire (fire horns and strobes announcing the fire to people inside a building as well as well as calling the fire department).
There's also a fire control system that sometimes suppresses the fire and limits the spread of smoke.
Basically, all of the fire alarm system works as a chain to detect fire and let people know about fire. If something in the chain doesn't work, if one of the links is broken, some or all of the fire alarm system doesn't work. If you're one of those that didn't get warned of a fire, for you, a broken link in the fire alarm system makes the whole system useless.
The Whole System has to Work
In a fully conventional fire alarm system, devices like smoke detectors, pull stations, fire horns, strobes, etc. are really dumb devices; they have to be tested individually on a regular basis to see if they perform as they're supposed to.
The fire alarm control panel has a green light on it, and it has yellow lights on it to let someone know if it is not working.
However, the wiring is not only unseen behind walls and ceilings, but by itself the wiring does not have any way of showing that it is not working correctly; if a wire breaks, or a connection comes loose behind a device or in a junction box somewhere, no none will ever know about the problem until there's a fire.
If a smoke detector or a fire horn goes bad, a small area of the building quits working. No one is supposed to count on this, but usually a device close by can take up the detection or announcement of the fire.
The wire is different. If a wire goes bad, a whole section of the building quits detecting or announcing, nothing is available close by to take up the slack.
The wire, even though it is hidden in the wall and ceiling, has to be constantly checked to make sure it will carry the all-important alarm signals.
In all this, keep in mind that the wire itself doesn't detect fire, the wire itself doesn't announce to anyone that there's a fire, the wire itself doesn't control anything else in the fire alarm system. The wire only carries signals to and from the devices and control panel.
Continuity Testing of the Wires
Now we get to the method of checking the wire to make sure it's connected to everything in a conventional fire alarm system.
The Fire Alarm Control Panel (FACP) checks the wires. It does the checking by running a small amount of electrical current through the wires to see if the wires are connected. In other words, the FACP is constantly running a continuity check of the wires to make sure they are always connected.
Electricity, though, requires a complete electrical path in order to flow around in a circle from the power source (the fire alarm control panel) through all the wire, and back to the power source. If the wire is not continuous, the flow of electricity just stops.
The pair of wires in a circuit cannot just end, though. Even at the end of the circuit there has to be a continuous electrical path, or the electricity won't flow around the circuit.
End of Line Resistor
That's where the end of line resistor comes in. It completes the electrical path so electricity will flow around the circuit, the control panel will detect the electricity flowing around the circuit, and people will know that the wires are still complete between the devices and the control panel.
Limited Electrical Path
The question in the back of minds is "Why not just connect the wires together at the end of the circuit, that would make a complete electrical path, wouldn't it?"
Yes, connecting the wires together at the end of the circuit would make a complete electrical path, but that's also a short circuit. Connecting the wires together is an electrical short allowing an unlimited amount of current to flow.
It's like what a pull station in alarm does on an input circuit, or a shorted fire horn does on an output circuit.
The end of line resistor is a current-limiting device; the end-of-line resistor allows a small amount of current to flow so the fire alarm control panel can detect that the wires are continuous. However, the end of line resistor does not short out the circuit, the end of line resistor allows the detection or announcement signals to be sent.
This continuity checking by the fire alarm panel is just one method used by the control panel to keep the whole chain of the fire alarm system working.
The End-of-Line Resistor is at the End of the Circuit
The whole purpose of continuity checking the continuity of the wires is to check the continuity of ALL of the wires. The only place that the end-of-line resistor is able to allow all of the wires to be checked for continuity is at the end of the circuit.
The circuit can't be T-tapped either because there cannot be an end-of-line resistor at the end of a T-tapped circuit, at least not without compromising the fire alarm system.
Conventional Class B or Class A
Electrically, there's almost no difference between a conventional Class B circuit and a conventional Class A circuit.
Electrically with Class B, the end of line resistor is located somewhere in the building, electrically with Class A, the end of line resistor is located on the circuit board of the control panel.
For the continuity checking of either a Class B or a Class A circuit by the control panel, there is no other electrical difference.