Advance Beneficiary Notice – The ABN is a written notice you must issue to a Fee-For-Service beneficiary before furnishing items or services that are usually covered by Medicare but are not expected to be paid in a specific instance for certain reasons, such as lack of medical necessity. The ABN allows the beneficiary to make an informed decision about whether to get the item or service that may not be covered and accept financial responsibility if Medicare does not pay. If the beneficiary does not get written notice when it is required, he or she may not be held financially liable if Medicare denies payment, and you may be financially liable if Medicare does not pay. The ABN is used for Medicare Part B (outpatient) and Part A (limited to hospice, Home Health Agencies, and Religious Nonmedical Health Care Institutions only) items and services.
Further reading
https://www.medicare.gov/claims-and-appeals/medicare-rights/abn/advance-notice-of-noncoverage.html