45 CFR § 162.412
Implementation specifications: Health plans
April 14, 2021
(a) A health plan must use the NPI of any health care provider (or subpart(s), if applicable) that has been assigned an NPI to identify that health care provider on all standard transactions where that health care provider's identifier is required.
(b) A health plan may not require a health care provider that has been assigned an NPI to obtain an additional NPI.