According to Masterline Logistics, a container seal is a device inserted into the handle assembly on the door of the container, numbered and coded and referenced on the bill of lading and manifest.
Container seals typically look like this picture, though there are many varieties of seals. The important thing to note is that container seals have a series of numbers and/or letters on them. This code is known as the seal code and is extremely important.
So why is the container seal code important?
According to The World of Shipping, whoever receives the container must make sure that the seal code shown on the bill of lading exactly matches the seal code physically on the container.
If the bill of lading and the physical seal do not match, the shipper needs to be notified immediately. It is possible that the wrong freight is being delivered and an investigation by the carrier and the company responsible for the initial shipment needs to take place.
A broken or damaged seal indicates that the cargo in the container may have been tampered with. The sophistication of seals can range from a simple, numbered plastic tag to an electronic seal that immediately indicates through global electronic positioning technology when and where it has been broken.