Since MFD is dispatched via computer/landline at the station, the "Car 300, Engine 25 responding to the hazmat incident..." announcement on the radio is a verbal means that alert all other MFD units on the air that 1.) there was a call 2.) who is going 3.) where they are going and 4.) they are running "hot" so watch out for them.
I'm sure Car 300 is an actual unit, but I'm not into MFD scanning so I haven't the foggiest idea who/what it is.