The government is the biggest payer in the United States via Medicare, Medicaid and Tricare.
Tricare covers military members, retirees, and their families.
Medicaid covers low income, children, blind, disabled, children, and those eligible for federal income assistance payments. It is funded by both federal and state governments.
Medicare covers people 65 and older, blind, disabled, and those with ESRD or permanent kidney failure. It is solely federally funded.
Medicare has four parts, each covering different types of medical services:
A - Inpatient hospital, SNF, Hospice Care, Home Health Care
B - Outpatient, Medically necessary provider services, some medical and preventative services A doesn't cover. It is an optional coverage and usually requires a co-pay and a premium.
C - Combines A and B, also can include D. These are known as medicare advantage plans, they can be PPO/HMO ect. They are managed by third party insurers. These parties are approved by medicare.
D - Prescription medications. All medicare recipients have this program. It is also run by approved third parties.
Medicare guidelines are often adopted by third party payers - but they don't have to be.