I understand when Medicare is primary and when Medicare is secondary. What is the difference in coverage between a Medicare Supplement and Medicare as a secondary insurer?

If you have a Medicare Supplement then the provider will bill Original Medicare first. At that time Medicare will pay the allowable amount and then return an explanation of benefits stating the beneficiary's portion. Based on the Medicare Supplement Plan that is in place (A-N) the Medicare Supplement will pay a portion or all of the remaining amount due. If they pay only a portion based on the plan (A-N), then according the plan guidelines, the beneficiary would pay any outstanding amount at that time.

If a Medicare beneficiary is covered on a employer or retiree group plan and due to the size of the plan, the group plan is primary, then the group plan benefits will apply first and any amounts due by the Beneficiary will be billed to Medicare second. If it is a Medicare covered service, then Medicare will pay the remaining amount due as the secondary payor up to the amount allowed by Medicare. If the service is not allowed by Medicare, than the beneficiary's co-insurance or co-payment under the group plan would be their responsibility.