NPA-NXX codes refer to the combination of area codes (NPAs) and local exchanges (NXXs). The combined code may contain up to 10,000 telephone numbers (the last 4 digits) that are usually located within a specific geographic region associated with the Cental Office of that code.
Example: 212-555 is the NPA-NXX for the phone number 212-555-1212
NPA-NXXs provide a mechanism for geocoding direct mail lists, particularly for the telecommunications industry.
See also CLLI Code.