Why are tank vehicles required to have specific markings or specifications?

Cargo tank specifications are critical safety information. DOT MC 306 / DOT 406 tanks carry low-pressure flammable liquids like gasoline; MC 307 / DOT 407 carry chemicals and acids; MC 312 / DOT 412 carry corrosives; MC 331 carry compressed gases under high pressure; MC 338 carry refrigerated liquefied gases. Each spec has different design pressures, material compatibility, and operational rules. A tank cannot legally carry a material outside its specification class. The spec is stamped on a metal data plate on the tank and is part of the vehicle's permanent identity. Driving a tank with the wrong material is dangerous and illegal — chemistry can react with the tank itself.