Three blades:
A Helicopter Emergency Medical Service (HEMS) flight is for
immediate and
rapid transportation of medical personnel, supplies (equipment, blood, organs, drugs) or ill or injured persons and anyone else directly involved. An approval is required. The purpose of a casevac is to give immediate assistance to sick or injured people in life threatening circumstances, typically from the scene of an accident.
Having said that, I'm not sure if it helps, but there is a subtle difference
Phil