A volume damper, VD, is a tap inside a duct. Balancing engineers use them to push air around the system until every terminal delivers its design flow. They live in awkward places, above ceilings and inside shafts, so the difference between a good one and a cheap one is whether the mechanism still turns at the first rebalance, three years later.
A fire damper, FD, has one job: when heat arrives, a fusible link or actuator slaps the blades shut and the duct stops being a road for fire and smoke. It is a life-safety product. It is certified, tested, and absolutely not the line item to value-engineer. In a wall or floor penetration with a fire rating, the code is not ambiguous about needing one.
MFD, the multi-blade damper, is the industrial sibling: parallel blades for isolation and control in larger ducts, often with motorized actuators tied into the building management system. The valve the plantroom uses to breathe.