The history of classes and styles of circuits is interesting. Back in 1987, a proposal was accepted to start using Styles of Circuits instead of the Class A and B we were all used to. Well, at least some thought so.
The fire alarm systems industry has changed considerably over the years. Many old-timers can remember a fire alarm system that only detected a fire and sounded an alarm. Nonfire alarm systems did not interface with these fire alarm systems.