A seal is a numbered security device — often a plastic or metal tag — locked onto a trailer’s doors after it’s loaded. Its unique number is recorded, so anyone along the way can tell whether the trailer has been opened since it was sealed.
An intact seal with a matching number is assurance that no one tampered with the cargo in transit. The number is typically noted on the bill of lading, and the consignee checks it against the paperwork before breaking the seal to unload.