Holds the system at its normal standing pressure by making up minor leakage, so the main fire pump does not start unnecessarily. It plays no part in fighting a fire and is not sized to. Its entire purpose is to keep the main pump from cycling on gradual pressure decay.

Because if it could keep pace with a single open sprinkler head it would mask a real flow and prevent the main pump ever starting. It is deliberately undersized relative to the fire duty, and specifying a larger one to be safe defeats the protection the system depends on.

The jockey starts and stops above the main pump's start pressure, so it responds first to any small decay. The main pump only starts when demand exceeds what the jockey can supply. Settings too close together cause the jockey to short-cycle or the main pump to start unnecessarily.

Almost always a system leak large enough that the jockey cannot restore pressure and settle, or pressure settings with too narrow a differential. Short-cycling is a symptom worth investigating rather than adjusting away, because it usually indicates a leak that will eventually matter.

Usually a small vertical multistage unit, since it must produce the system's full standing pressure at a very small flow - which is exactly what a multistage pump does well. It takes little floor space, which suits a pump room already occupied by the main set.

Not universally, but it is standard practice on any system where gradual leakage would otherwise cause nuisance starting of the main pump. Whether it is mandatory follows the governing code and the system design, but omitting one generally causes operational problems rather than saving anything.

Yes, a separate jockey pump controller with its own pressure switch and settings, distinct from the main fire pump controller. It handles automatic start and stop on pressure, and its settings are what establish the relationship with the main pump's start point.