Basically its a vehicle that is crewed by 6 personnel and Carries Heavy Rescue gear ALS kits and water with a foam solution, the reason for this is because most countries have abolished Heavy Rescue Vehicles. over in my local area these are normally about 3-4 thousand more than a normal pump and tend to go to almost every call.
I know that we have have Heavy Rescues and rescue engines but heavy rescues as i said are not used by most service anymore at least in the UK that is. and the rescue engine needed to be the rank of at least captain which is a bit annoying because most engines are rescue engines or Rescue Pumps in the UK.
Another Vehicle is a bulk Foam Lorry AKA Foam Tanker - this is basically a vehicle that carries large containers of foam to an incident i think this would be a good vehicle because most fire fighting appliances/apparatus Carrie a small amount of foam and most large industrial fire cant be extinguished until all the needed foam is on scene for obvious reasons adding not only more realism but more equipment and a large amount of calls could also be add that require the for example Refinery fires.