When a Medicare provider (doctor, lab, etc.) signs a contract to provide services for Medicare, they can become either preferred or not.  The preferred provider accepts "Medicare Assignemnt", meaning, they will accept the price Medicare allows.  Say, if the bill is $100, Medicare will pay typically 80% (after the deductible) or $80.  The doctor will wait, and Medicare will pay the $80, and he will charge you the $20, or wait for your Medicare Supplement to pay.  Some primary care doctors, and many specialists do not take assignment.  Therefore, the can charge more, known as the "excess charge".  Law permits them to charge no more than 15% above the "assignment rate."  This is the excess charge paid at 100% by plans F, I and J.  Plan G would pay 80% of this amount.