For Entry Summary Declarations purposes, a container is a sealed, reusable, rigid unit of transport equipment designed to hold goods for intermodal transport, allowing them to be transferred between different modes (ship, rail, truck) without intermediate reloading.
For Entry Summary Declarations where goods are transported in a registered shipping container, the container identification number must be of the format four letters followed by seven numbers (for example, ‘ZZZZ9999999’).
For customs declarations, details on containers are available in Container Identification Number for Imports on GOV.UK.