42 CFR §424.53

1. (a)Emergency inpatient and outpatient services furnished by a nonparticipating U.S. hospital that has not elected to claim payment in accordance with subpart G of this part.
2. (b)Certain medical and other health services covered under Medicare Part B and furnished by a nonparticipating U.S. hospital, if the hospital does not receive assigned payment as a supplier under § 424.55.
3. (c)Emergency or nonemergency services furnished by a foreign hospital if the hospital does not have in effect an election to claim payment in accordance with subpart H of this part.
4. (d)Physician and ambulance services furnished outside the United States.
5. (e)Services furnished by a supplier if the claim has not been assigned to the supplier.