The word "Priority" is used for "When two messages come in at the same time, which one should the panel handle first?" In small fire alarm systems, priorities don't mean much; in large fire alarm systems, priorities are quite important.
Priorities of the Fire Alarm System
The purpose of a fire alarm system is to detect fires, and, if there's a fire, warn people of fires. That's a highest-level priority signal; when there's a fire, no one cares if a wire is broken. Even when a supervisory signal is there, like a valve is closed, it's still more important to get people out of the building than the look for a closed valve. Once the fire alarm is properly sounding off, then the alarm system can look at other, lower-priority type signals.
Secondary to a fire alarm, the fire alarm system is watching other systems connected to the fire alarm system, like the sprinkler system. In other words, the fire alarm system supervises other systems. When a valve on the sprinkler system is closed, people aren't warned of fire, but someone should be told to open the valve again. Once the panel has turned on the panel's local buzzer and the supervisory light, then the panel can look at other, lower-priority type signals.
Yes, it's important to make sure the fire alarm system itself is properly working. Once the alarms are sounding (highest priority), and the supervisories are displayed on the panel, then the panel can deal with the trouble signals, like broken wires or dirty smoke detectors. After the panel has looked at the alarms, the supervisories, and the troubles, the panel can take care of the lowest-priority signals.
Most fire alarm panels don't have a monitor priority. If the panel does have a monitor priority, the monitor signals are used for things like turning on or off non-emergency equipment. EST is one of the few manufacturers that seem to even have the low-priority monitor signals.
Do Priorities Matter?
On smaller fire alarm systems, ones that have a few dozen detectors and horns, the panel will take care of all fire alarms, supervisory alarms, trouble activations, and monitor signals in a fraction of a second. Because there are so few devices, priorities really don't make a big difference when the systems are small.
On larger systems, systems that have thousands of detectors, cover several buildings, use any one of hundreds of voice announcements, have many hundreds of horns and strobes, and are there are dozens of fire zones, priorities are very important.
Code Blue Alarms in hospitals, for instance, have to be at the full fire alarm priority level. Code Blue Alarms are where a person has quit breathing, and a "Crash Cart" along with trained medical personnel have to arrive within 3 minutes, or the person is dead.
If in a large fire alarm system, the Code Blue Alarm is sent as a supervisory level alarm, the Code Blue announcements can easily take 20 seconds to get through the system and heard by medical personnel. If, however, in the same large fire alarm system the Code Blue Alarm is sent as a fire alarm signal, the Code Blue announcement might take less than a second to get through the system. Twenty seconds may mean the difference between life and death to someone.
Priorities Do Matter
With the speed of signal processing in smaller systems, priorities usually don't matter. However, the larger and more complicated systems with lots of devices and actions need to have different levels of priorities so that the panel can process the Fire Alarms, Supervisory Alarms, Trouble Activations, and Monitor Activations in order of importance.