Plain version: Part B is medical insurance for doctors and outpatient care. You pay a monthly premium and a yearly deductible, then usually 20% of each approved bill, and Original Medicare puts no ceiling on that 20%.
Costs:
Monthly premium: paid by all beneficiaries, even in months with no services. Enrollees whose modified adjusted gross income is over a threshold pay an income-related monthly adjustment amount (IRMAA) on top of the standard premium.
Annual deductible: paid once each year before Original Medicare starts to pay. (Part A's hospital deductible works by benefit period instead.)
Coinsurance, generally 80-20: after the deductible, Medicare usually pays 80% of the Medicare-approved amount and the beneficiary pays 20%. Example: a $500 approved amount means the beneficiary usually pays $100.
Benefits:
Medically necessary outpatient health and diagnostic services.
Physicians' and surgeons' services, in the office and in the hospital.
Home and hospice care not covered under Part A, as listed in the CDI outline. Note: current Medicare.gov lists home health under both Part A and Part B but hospice under Part A only, and 42 CFR §418.20 ties hospice to Part A.
No annual out-of-pocket maximum: Original Medicare has no yearly limit on Part B coinsurance unless the person has supplemental coverage such as Medigap or joins a Medicare Advantage plan. Example: $40,000 of approved charges after the deductible means $8,000 of coinsurance.
Approved vs. non-approved physicians:
A doctor who accepts assignment takes the Medicare-approved amount as payment in full, so the patient owes only the deductible and coinsurance.
A nonparticipating doctor who does not accept assignment may charge more, but in many cases no more than 15% above the approved amount (the limiting charge). Example: a $200 approved amount allows a charge of at most $230.