Here follows my understanding of Mayo+Medicare:
In 2015, Mayo AZ, did not "accept Medicare assignment". This meant that Mayo was able to charge patients 15% more than the Medicare Part B approved charges. However, this meant Mayo billed the patient and the patient submitted a claim to an insurer. If the patient had a medigap supplement F or G, the entire bill was paid after deductible. If the patient had, eg, medigap N, the patient had to kick in the 15%. If the patient had an 'advantage plan' (Part C), the bill may not have been covered - the patient was potentially responsible for the entire bill.

Now, Mayo is sending out this notice about Advantage plans which states that it "accepts" traditional Medicare. This is a bit ambiguous, unfortunately. The phrase "accept Medicare assignment" is the term of art that means: the provider will bill Medicare directly and accept the Medicare approved reimbursement as payment in full (ie, will not charge the 15% Part B excess). Of course the patient is responsible for any part of the Medicare approved reimbursement not covered by Medicare+medigap supplement, eg, deductibles, copays, etc.

The government site medicareDOTgov (as a newly joined poster, this site prevents my including links)
indicates that Mayo AZ does indeed "accept Medicare assignment". This is good to know because it means cheaper medigap supplements like N cover all Mayo charges: eg, in my location medigap G is about 35% more than N.

It would be great if Mayo would confirm my understanding. Unfortunately, a call to the customer service line rings & rings after wading through the menu.