Magnetic Door Locks and Magnetic Door Holders work the same way. When power is applied to the door lock or door holder, the electromagnet is turned on and keeps the door locked or the door held open.
Once a fire alarm occurs, the fire alarm panel sends a signal to the control relay
to activate. Because it is wired to turn off power when it activates, the control relay is used to turn off the door lock or door holder electromagnets.
Magnetic Door Lock
A magnetic door lock keeps the door locked. In a fire alarm system, though, the door lock has to work to unlock the door even when there is a power blackout for the neighborhood. If there is a fire, then, the magnetism is turned off when the fire alarm control relay turns off the power.
Magnetic Door Holder
A magnetic door lock keeps the door open so people can go through the door at all times. In a fire alarm system, though, the door holder has to allow the door to close even when there is a power blackout for the neighborhood. If there is a fire or smoke, then, the magnetism is turned off when the fire alarm control relay turns off the power.
The power for the door lock or the door holder is from the fire alarm control panel. The reason it comes from the control panel is so the power can be battery backed up in case of a local power blackout.
Sometimes, though, the power for door holders comes from a separate power supply. This extra power supply provides extra power when the fire alarm control panel doesn't have enough power in its batteries for a 24-hour blackout to run the fire alarm system, and also keep the doors locked or open.
The instruction sheets that come with the control relays for the fire alarm system describe the voltage and current ratings for the relay contacts. They also say something like "Pilot Duty" or "No More Than xxx Amount of Reactance" (whatever that means to a fire alarm installer).
What that really means is if there's a motor or electromagnet being turned on or off, the system needs a Buffer Relay to protect the fire alarm's control relay.
See Why Install an Extra Relay for a Door Holder?
This has instructions and diagrams for the added Buffer Relay.