In March 1927, the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: standard weight (STD), extra-strong (XS), and double extra-strong (XXS), based on the iron pipe size (IPS) system of the day. However these three sizes did not fit all applications. The committee surveyed the industry and created a system of schedule numbers that designated wall thicknesses based on smaller steps between sizes, although IPS and NPS numbers remain equivalent.
The original intent was that each schedule would relate to a given pressure rating, however the numbers deviated so far from wall thicknesses in common use that this original intent could not be accomplished. Also, in 1939, it was hoped that the designations of STD, XS, and XXS would be phased out by schedule numbers, however those original terms are still in common use today (although sometimes referred to as standard, extra-heavy (XH), and double extra-heavy (XXH), respectively). Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others.
If you have ever done any plumbing then you recognize the term – Schedule – as PVC pipe can be bought in Schedule 40 and Schedule 80 (at Lowes for example). Interestingly, we can take a look at 1 in. (Nominal) PVC/CPVC pipe. The O.D. is 1.315 in. with the I.D. different. For Schedule 40 the I.D. is 1.049 in. with a 0.133 wall thickness while the Schedule 80 is 0.957 I.D. with the walls 0.179 in. thick.
The important thing to take from this (as far as modeling is concerned) is that the 1.315 O.D. .. this is the NPS (Nominal Pipe Size) and is the same whether we are talking about a plastic PVC pipe or one made of cast iron.