42 CFR § 424.66 - Payment to entities that provide coverage complementary to Medicare Part B.
(a) Conditions for payment. Medicare may pay an entity for Part B services furnished by a physician or other supplier if the entity meets all of the following requirements:
(1) Provides coverage of the service under a complementary health benefit plan (this is, the coverage that the plan provides is complementary to Medicare benefits and covers only the amount by which the Part B payment falls short of the approved charge for the service under the plan).
(2) Has paid the person who provided the service an amount (including the amount payable under the Medicare program) that the person accepts as full payment.
(6) Identifies and excludes from its requests for payment all services for which Medicare is the secondary payer.
(b) Services paid for by the entity. An entity is not required to pay and claim reimbursement for all Part B services furnished to members of its plans. However, if it does not pay and claim reimbursement for all those services, it must establish in advance precise criteria for identifying the services for which it will pay and claim reimbursement.