Schedule pipe is determined by the ‘nominal bore’ and another non-dimensional number for wall thickness, referred to as the Schedule.
The term NB (Nominal Bore) is often used interchangeably with NPS (Nominal Pipe Size) and DN (Diameter Nominal). “Nominal” is a non-dimensional number that identifies the hole diameter.
To ensure correct supply, always specify at least 2 dimensions you require: Outside diameter (OD), inside diameter (ID), or wall thickness.