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. Lengths of containers, which each have a unique ISO 6346 reporting mark, vary from -foot (m) to -foot (m) and heights from -foot (m) to feet 6 inches (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 / teu) which is a unit of capacity equal to one standard (length × width) container.