I think we're confusing apples and oranges.
Fire alarm panels typically have brand-specific detectors, separate signalling devices, and are installed complete by the alarm contractor.
Your typical house, however, has independent alarm detector/ signalling units that are interlinked. These detectors work off of 120 volts, have a battery for back-up power, and are (these days) installed so that if one sounds, they all do.
These household detectors are often installed on their own dedicated circuit, using 14/3, with the third conductor used to "communicate" any alarm to other units.
Dedicated "fire alarm" cable is an entirely different thing, and is NOT used for line voltage or power applications.
Now, it is possible to tie household alarms into a central-station alarm panel- using a relay, that ties into the system in a manner just like another smoke detector. The alarm contractor should know exactly which parts to use, and all the EC needs to know is where to run the wire.
That said- and I mean no disrespect to the alarm trades- but alarm installers typically have a very limited, brand specific training. They have been taught one way, the only way for the brand they install, and sometimes get a little confused.