In 1995 an ISO agreement was reached concerning container marking codes that would depict the length, height, and type of container in a short and easy to read sequence. This sequence is composed of four letters or digits. The first element of the sequence is about the length of the container with the most common codes being 2 (20-foot container) and 4 (40-foot container). The second element is about the height of the container with 2 (standard height of 8 feet and 6 inches) and 5 (high cube container of 9 feet and 6 inches) prevalent. The third and fourth refer to the container type. G1 relates to a general container that has passive vents at the upper part of cargo space. R1 would be a mechanically refrigerated and heated container. U1 would be an open-top container with removable top members in end frames. P1 is a platform container with two complete and fixed ends. Finally, T1 is a tank container for non-dangerous liquids.
The most common container used for maritime shipping, a standard 40 footer, would have 42G1 as a sequence while the same container as a high cube would have a sequence labeled as 45G1 (see above figure). 22U1 is a standard 20-foot open-top container. 45R1 is the most common sequence for a reefer (40-foot high cube). The container width code is usually not used since the great majority of containers have a width of 8 feet, but there are pools of containers having a width of 8 feet and 2 inches that uses the E label. High cube 45-foot containers (LEG1) are convenient for intermodal transportation in Europe as they have a size that fits exactly 33 European pallets of 1.2 by 0.8 meters.