When are containerized loads used?

Containerized loads generally are used when freight is carried part way by rail or ship. Delivery by truck occurs at the beginning and/or end of the journey. Some containers have their own tie-down devices or locks that attach directly to a special frame. Others have to be loaded onto flatbed trailers. They must be properly secured just like any other cargo.
You cannot inspect sealed loads, but you should check that you do not exceed gross weight and axle weight limits.
[California Commercial Driver Handbook]