In international shipping, all containers must be secured with at least one seal before a shipping line will allow it to be shipped. A seal is a numbered tag that is secured around the lock of a shipping container.

A seal on a container cannot be opened without cutting it. Because it can only be removed once, the container will remain unopened until it is delivered to the buyer.

Each seal should be identifiable by a series of numbers and letters. This is called the seal number. When the container has reached its destination, the buyer should verify if the seal number is the same as the one listed in the Bill of Lading. The supplier is responsible for entering the correct seal number(s) during Shipping Instructions.