What is meaning of inch-dia in piping project ? Is it applicable for all pipe schedule of same diameter ?

For any given pipe size and varying schedule numbers or thicknesses its Outside Diameter (OD) remains constant and Internal Diameter (ID) varies. With increase in thickness the strength increases but its ID decreases. OD is kept constant to help the support hardware design so that the same support hardware can be used for same pipe size (varying thicknesses).