45 CFR § 162.406 - Standard unique health identifier for health care providers.
(a) Standard. The standard unique health identifier for health care providers is the National Provider Identifier (NPI). The NPI is a 10-position numeric identifier, with a check digit in the 10th position, and no intelligence about the health care provider in the number.
(b) Required and permitted uses for the NPI.
(2) The NPI may be used for any other lawful purpose.