45 CFR §162.610

1. (a)The standard unique employer identifier of an employer of a particular employee is the EIN that appears on that employee's IRS Form W-2, Wage and Tax Statement, from the employer.
2. (b)A covered entity must use the standard unique employer identifier (EIN) of the appropriate employer in standard transactions that require an employer identifier to identify a person or entity as an employer, including where situationally required.
3. (c)Required and permitted uses for the Employer Identifier.
   1. (1)The Employer Identifier must be used as stated in § 162.610(b).
   2. (2)The Employer Identifier may be used for any other lawful purpose.