A pipe schedule is the wall-thickness step of a pipe size: since every nominal size keeps one fixed outside diameter, moving from Sch 40 to Sch 80 to Sch 160 grows the wall inward and shrinks the bore. The usable inside diameter is computed, not looked up — ID = OD − 2 × wall — and that subtraction is exactly how a fabricator works from the published OD and wall tables of ASME B36.10M for steel or ASTM D1785 for PVC.
The system carries historic quirks worth knowing. Sch 40 coincides with the older “standard weight” (STD) designation only through NPS 10 — at NPS 12 they part ways, STD holding a 0.375 in wall while Sch 40 is 0.406 in. Sch 160 is simply not produced below NPS 1/2 or at NPS 3-1/2. And PVC walls under ASTM D1785 are specified as minimums with plus-side-only tolerance, so an ID computed from the minimum wall is the maximum bore — real average bores run slightly smaller.
Not every material speaks schedule: copper water tube grades its walls by type letter instead, with Type K the heaviest (underground service), L the standard for pressurized plumbing, and M the lightest for water distribution where codes allow it.