Here
https://www.selinc.com/printProduct.aspx?id=1016, Now you know what they are.
I'd like to make a few comments. In "typical" fire alarm systems, a resistor is placed across an open contact. Thus, the fire alarm panel can determine if the wire has been cut.
I made a simple annunciator out of 3PDT relays as part of a safety panel. They generally operated with a brief momentary pulse and the relay would electrically latch. That's actually fine and dandy unless you want to know when the alarm clears. So, in the case of a fire alarm activation of this home made annouuniator, I added an LED which went into the SPDT relay of the fire panel.
The design was, the red LED indicated that there was continuity between the Common and NC terminals of the fire alarm panel, so it was on when it was not active. The LED would go out, if the fire alarm was reset. The other part of the announciator used IDEC labelable rectangular LED indicators (expensive) and indicated an alarm.
So, I did not have the tradidtional "monitored contact", but the system would know if an entire wire got severed.
The problem with alarms is that you need to latch them and you need to know their current status. In the case of a fire or safety alarm, contact monitoring is employed which is done with a resistor across the open contact or a two level current trip. The trip needs to be latched and ALSO monitored for currrent conditions.
The 3PDT relay basically used the contacts as follows:
1) Alarm indication
2) Latch the relay
3) what I would call the alarm loop. All of these contacts would be in series.
A power fail required the panel to be reset, which could be a nuesance in your applicaton. So, lots of things to think about.