well first off the RAC units are dispatched to all fires and basically are a big water cooler. they supply hydrating beverages for ff's at the job. In the summer they have wet towels to cool off with. The MERV if im not mistaken show up to all 2nd alarms and greater. It is basically a huge ambulance that is the size of a coach bus. They can check you CO levels in your body to treat injuries. They can treat several patients at once. So think of the merv as a huge ALS ambulance that can treat mulitple patients inside.