Is the boosting to raise the sprinkler pressure so as not to trip the alarm or flood a dry sprinkler system from fluctuation in the municipal water pressure? Because that is the normal function of a booster pump.
Any water pump which takes suction from a public supply is generally considered a booster pump; however, the important distinction is whether or not the pump in question is a dedicated pump for the water-based fire protection system.......if the answer is yes, then NFPA considers the pump a fire pump (regardless of the source).
The legal aspect of pumps, and the acceptable arrangements, also changes depending on the system design (NFPA 13 or NFPA 13R or NFPA 13D)..........your options change depending on the applicable standard. Research and time my friend.
There are pressure maintenance pumps in service. They are installed parallel to the system supply, and do not affect system operation. They used to be used to boost the system pressure downstream of the system check valve. Their purpose was to maintain system pressure higher than a fluctuating supply pressure. One of the monitoring companies here used to install them to avoid false alarms.
Now when technicians see them, they don't know what they are for. Most of the time a a flow switch with a delay has been installed, and the old pump is left in-place. I think they are left "just in case" because most people don't even know what they are.