If you've been shopping around for PVC you may have heard the term “schedule”. Despite its deceiving title, schedule doesn't have anything to do with time. A color pvc foam board’s schedule has to do with the thickness of its walls. Maybe you've seen that schedule 80 pipe is slightly more costly than schedule 40.
Though the outside diameter of a schedule 80 pipe and a schedule 40 pipe are the same, 80 pipe has thicker walls. This standard of measuring pipe came from a need to have a universal system for referring to PVC. Since different wall thicknesses are beneficial in different situations, the ASTM (American Society for Testing and Materials) came up with the schedule 40 and 80 system for classifying the two common types.