Geographic Locator (Region / Subregion)

What is a Region / Subregion?    The world is divided into nine regions; each corresponding to the geographic limits of one of the nine regions in the NIMA Catalog of Hydrographic Products.  As shown in the graphic below, each Region is further divided into numbered Subregions. The first digit of all five-digit chart numbers indicates the geographic region while the first two digits of all five-digit chart numbers refer to the geographic subregion.