Water Tender
Apparatus built to haul large volumes of water to incidents where there are no hydrants. Often called a tanker outside wildland use.
A tender is a rolling water supply. Where there is no hydrant system, water has to be trucked to the fire, and tenders shuttle it from a fill site to the scene.
This is a rural and suburban fixture rather than an urban one, and a good example of fleet composition following the served area rather than the population. A county with unhydranted districts needs tenders; a dense city does not.
The fleet characteristic that matters is that a large volume of water is a large moving mass. Tenders are heavy and their handling changes as the tank empties, with real consequences for driver training, tire and brake wear, and rollover risk. Both the safety program and the maintenance schedule should treat them as their own class rather than as another truck.