Which of the following best defines a fire service main?

A fire service main is defined as a pipe and its appurtenances located on private property that is used to supply water for firefighting purposes. This system is separate from the public water supply and is specifically designated for delivering water to fire protection systems, such as sprinklers or standpipes, within a building or on the premises. By focusing on private property, this definition emphasizes that fire service mains are not built for general public use, but rather for protecting property from fire hazards.

The other options refer to other types of water supply systems. For instance, a main supply line for public use is related to municipal water distribution that serves the general population. A fire hydrant system is a component of a public water supply system that provides access to water for firefighting, but it does not encompass the entirety of what constitutes a fire service main as it pertains specifically to on-site systems. Lastly, a domestic water supply line typically refers to the infrastructure that provides potable water to residences or commercial buildings, which serves a different purpose from the dedicated fire protection needs that fire service mains address.