This was the wording on page 354 of The Doctor's Guide to Concierge Medicine. I believe it is a typographical error:
Experts, writers, doctors and attorneys recommend that Concierge Medicine and D.P.C. physicians set up a separate business corporation alongside their professional corporation if they choose to bill insurers. These types of physicians should distinguish between the covered services for which they are collecting a fee, and the uncovered services for which they are billing insurance. Physicians are encouraged to develop a menu of fees and services to offer.
I understood the the retainer fee (or monthly, quarterly or annual membership fee) was to provide for services that are NOT COVERED. And, that collecting any fees would be only for COVERED SERVICES that the insurance would pay for upon receiving a bill.