It's in the CDL manual. This is why they ought to keep the Class A license requirement for towing heavy trailers. There is no way people are going to know this info exists otherwise.
"Cargo Tiedown
On flatbed trailers or trailers without sides, cargo must be secured to keep it from shifting or falling off. In closed
vans, tiedowns can also be important to prevent cargo shifting that may affect the handling of the vehicle.
Tiedowns must be of the proper type and proper strength. The combined strength of all cargo tiedowns must be
strong enough to lift one and one-half times the weight of the piece of cargo tied down. Proper tiedown equipment
must be used, including ropes, straps, chains, and tensioning devices (winches, ratchets, clinching components).
Tiedowns must be attached to the vehicle correctly (hook, bolt, rails, rings).
Cargo should have at least one tiedown for each ten feet of cargo. Make sure you have enough tiedowns to meet
this need. No matter how small the cargo, it should have at least two tiedowns holding it."