One circuit type tells the panel a fire exists. The other is how the panel tells everyone else. They run on the same system, at the same time, in opposite directions.
A fire alarm control panel (FACP) has to do two fundamentally different jobs: find out that something is wrong, and then make sure people know about it. SLC (Signaling Line Circuit) handles the first job — it carries two-way digital communication out to addressable initiating devices like smoke detectors, heat detectors, and pull stations, each with its own unique address, so the panel can poll them and know exactly which device, in exactly which location, is reporting an alarm or trouble condition. NAC (Notification Appliance Circuit) handles the second job — once the panel has decided an alarm condition exists, it sends power and signal out through the NAC to horns, strobes, speakers, and horn/strobe combination devices, activating every notification appliance the circuit serves.
These aren't two flavors of the same wiring, distinguished by which devices happen to be plugged in. They carry information in opposite directions for opposite purposes — one is how the panel learns, the other is how the panel responds. A real system runs both, at the same time, doing two complementary jobs neither one can do alone.