What is Grievance?
A complaint about the way your Medicare health plan or Medicare drug plan is giving care. For example, you may file a grievance if you have a problem calling the plan or if you're unhappy with the way a staff person at the plan has behaved towards you. However, if you have a complaint about a plan's refusal to cover a service, supply, or prescription, you file an appeal.
reference: Glossary of Medicare program