An intermodal container (also container, freight container, ISO container, shipping container, hi-cube container, box, conex box and sea can) is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system. "Intermodal" indicates that the container can be moved from one mode of transport to another (from ship, to rail, to truck) without unloading and reloading the contents of the container. Lengths of containers, which each have a unique ISO 6346 reporting mark, vary from 8 feet (2.438 m) to 56 feet (17.07 m) and heights from 8 feet (2.438 m) to 9 feet 6 inches (2.9 m). There are approximately seventeen million intermodal containers in the world of varying types to suit different cargoes. Aggregate container capacity is often expressed in twenty-foot equivalent units (TEU) which is a unit of capacity equal to one standard 20 ft × 8 ft (6.10 m × 2.44 m) (length × width) container.