A carrier needs a cargo tank built to the DOT 407 specification. What design requirement does 49 CFR 178.347 set that distinguishes it from a DOT 406 tank?

49 CFR 178.347 (DOT 407 cargo tank specification)

The jump from a DOT 406's low-pressure oval shell to a DOT 407's circular, higher-pressure design reflects the more demanding chemical cargoes it carries. Matching the right tank specification to the right commodity is a core part of what shippers and carriers must get right before a load ever moves.