Medicare Supplement plans are sold by private insurance companies and used to fill the gaps in Original Medicare (They are also known as Medi-Gap plans). Most of these plans allow you to use any doctor or facility that accepts Medicare. These plans only offer Medical coverage so they must be combined with a stand-alone prescription drug plan. Plans are standardized with the letters A-N.
As of January 1, 2020, Plans C and F are only be available for purchase for those who went onto Medicare before 1/1/2020.