A circuit vent is defined as:

A circuit vent is specifically designed to serve multiple fixtures that are located on the same branch of the plumbing system, typically when those fixtures are situated close to one another, such as in a bathroom. This type of vent allows for better air flow and pressure management within the plumbing system by connecting multiple fixtures to a single ventilation pathway.

Option B correctly states that a circuit vent serves two or more floor outlet fixtures, making it possible to adequately vent multiple drainage points without needing to install a separate vent for each one. This not only simplifies the plumbing layout but can also improve the efficiency of the system by reducing the potential for negative pressure that could lead to poor drainage and gurgling sounds from the fixtures.

In contrast, other options refer to specific types of vents or usages that do not encapsulate the function of a circuit vent appropriately. A vent for one fixture only cannot accommodate multiple outlets; a main vent for newly constructed buildings pertains to the primary means of venting a system and does not capture the essence of servicing multiple fixtures simultaneously; and a vent used for hot water systems would not accurately describe the function of a circuit vent, as this vent is focused on drainage rather than specifically for hot water systems.