inside diameter - plumbing pipe is measured in ID (inside diameter), example a 3/4 inch pipe has a inside diameter of 3/4 inch it is a plumbing pipe. Refregeration measures OD (out side diameter) for instance if the od measures 3/4 the pipe is not plumbing but refregeration.
Don't matter cast iron, pvc, copper, drainage or supply, sweat, leaded, threaded or glued. ID measurements are plumbing
