Two of the knowledge-only endorsements change how a vehicle behaves rather than what it is carrying, and both exist because the handling is genuinely different. For double and triple trailers, § 383.115 requires knowledge covering the procedures for assembly and hookup of the units; the proper placement of the heaviest trailer; and the handling and stability characteristics — the section names off-tracking, response to steering, sensory feedback, braking, oscillatory sway, rollover in steady turns, and yaw stability in steady turns. It also requires knowledge of the potential problems in traffic operations, including the problems the vehicle creates for OTHER motorists: slower speeds on steep grades, longer passing times, the possibility of blocking entry to freeways, splash and spray, aerodynamic buffeting, view blockages and lateral placement. That last group is unusual and worth noticing — the rule asks a driver to know the difficulties they impose on people around them, not only the ones they face.

For tank vehicles, § 383.119 requires knowledge covering the causes, prevention and effects of cargo surge on vehicle handling; the proper braking procedures when the vehicle is empty, full and partially full; the differences in handling between baffled or compartmented tank interiors and non-baffled ones; the differences in tank vehicle type and construction; and the differences in cargo surge for liquids of varying product densities. Surge runs through nearly every item, which is the point: a partly full tank is a moving load, and the braking behavior of the same truck changes with how much is in it.

Both sections end with a catch-all — § 383.115(e) covers operating practices and procedures not otherwise specified — so the enumerated topics are the floor rather than the ceiling. Neither endorsement requires a skills test under § 383.93(c), which makes them the two most commonly added to a license a driver already holds. That is not a statement that they are easy; it is a statement about what the test measures.