What means 15NB pipe?

The term "15NB" refers to a nominal bore size of a pipe, specifically indicating that the pipe has an approximate internal diameter of 15 millimeters. "NB" stands for "Nominal Bore," which is a standard measurement used to describe the size of pipes in Plumbing and engineering applications. This designation helps in identifying the size of the pipe for fittings and connections, ensuring compatibility in installations.