There is no central government agency with the role of assigning, monitoring, or approving NAICS codes for establishments.
NAICS Codes are self-assigned based on the establishment’s primary activity (the activity that generates the most revenue for the establishment)
Individual establishments are assigned NAICS codes by various agencies for various purposes using a variety of methods. The U.S. Census Bureau has no formal role as an arbitrator of NAICS classification.
The U.S. Census Bureau assigns one NAICS code to each establishment based on its primary activity (the activity that generates the most revenue for the establishment) to collect, tabulate, analyze, and disseminate statistical data describing the economy of the United States.
Generally, the U.S. Census Bureau’s NAICS classification codes are derived from information that the business establishment provided on administrative, survey, or census reports. (e.g. when a company applies for an Employer Identification Number (EIN), information about the type of activity in which that business is engaged is requested in order to assign a NAICS code).
Various other government agencies, trade associations, and regulation boards adopted the NAICS classification system to assign codes to their own lists of establishments for their own programmatic needs. If you question the SIC or NAICS code contained on a form received from an agency other than the U.S. Census Bureau, you should contact that agency directly.