By Luis Lopez, AI transportation consultant, CEO of Go Hub.io Holdings Corp and subsidiaries, and host of the Freight Guru Podcast
A seal is one of the cheapest pieces of equipment in freight and one of the most important pieces of evidence. Trailer and container seals do not stop a determined thief, and most are not designed to. What they do is show whether the doors were opened between the point where the seal was applied and the point where it was removed. That record matters for cargo security, for customs, and for any shortage or damage claim that follows.
This guide covers the main types of seals, what ISO 17712 means for ocean containers, and the checks shippers, drivers, and receivers should make at each step.
What a seal actually does
A seal is a tamper-evident device. Each one carries a unique number, and once it is closed it cannot be opened without visibly breaking or damaging it. When the number on the seal at delivery matches the number recorded at pickup, and the seal is intact, everyone has reasonable evidence that the load was not entered in transit. When it does not match, or the seal is damaged, that is a signal to stop and document before anything else happens.
Seals are only as useful as the records around them. A seal number that was never written down proves very little.
Common types of seals
Plastic seals
Plastic pull-tight or padlock-style seals are light, inexpensive, and easy to apply and remove by hand or with simple cutters. They are indicative seals: they show tampering but provide little physical resistance. They are common on domestic dry van loads, on interior compartments, and in situations where the main goal is a clean audit trail.
Cable seals
Cable seals use a length of steel cable threaded through the door hardware and locked into a body. They resist casual tampering better than plastic, fit door hardware of different shapes, and require cable cutters to remove. Their strength varies a lot by cable diameter and design.
Bolt seals
Bolt seals use a steel bolt pushed through the door locking hardware and snapped into a locking cylinder. They require bolt cutters to remove and are the standard choice for ocean containers and for higher-value trailer loads. Many bolt seals are manufactured to meet the high-security classification of ISO 17712.
ISO 17712 and ocean containers
ISO 17712 is the international standard for mechanical freight container seals. It sets test requirements and classifies seals by physical strength, with the highest category designated as high security. It also addresses tamper evidence and the manufacturer’s processes for controlling seal production and numbering.
For international ocean freight, and particularly for containers moving into the United States, shippers are generally expected to use seals that meet the ISO 17712 high-security classification. Supply chain security programs also emphasize seal control: storing unused seals securely, limiting who can issue them, and keeping a log of which number went on which container. If you load export containers, confirm with your ocean carrier or forwarder what seal requirements apply to your shipment before loading day, and buy seals that are marked and certified to the standard rather than generic hardware.
Our checklist for inspecting an export container before loading covers the door hardware checks that should happen before the seal ever goes on.
Recording the seal number on the bill of lading
The seal number belongs on the bill of lading, written clearly, at the moment the seal is applied. A good practice at the shipping dock:
- Finish loading and close the doors.
- Apply the seal through the locking hardware, not just a handle that can be removed with the seal still attached.
- Pull on the seal to confirm it is locked.
- Read the number off the seal itself and write it on the bill of lading. Do not copy it from a packing list or a pre-printed form.
- Have the driver confirm the number before signing.
- Photograph the sealed doors with the number legible.
If the driver was not allowed to watch the loading, the bill of lading often notes that the load was shipper loaded and counted. That notation has real consequences in a claim, which we cover in shipper’s load and count liability. For the document itself, see what a bill of lading is and how to fill it out.
Photos that actually help
Seal photos are cheap insurance. Useful photos show the seal number clearly, show the seal attached to the door hardware, and include enough of the trailer or container to identify it, such as the unit number. Take them at sealing, at any point in transit where the seal is inspected, and at delivery before the seal is cut. Timestamped photos stored with the shipment record are far easier to use later than images scattered across phones.
What drivers should check
- Confirm the seal number on the bill of lading matches the seal on the doors before leaving the shipper.
- Check the seal at stops, especially after the truck has been parked unattended.
- Never cut or replace a seal without instructions, unless law enforcement or an authorized inspector requires it.
- If an inspector breaks the seal, get the new seal number applied, and record who opened it, when, and why.
Broken or mismatched seals: what to do
If a seal is missing, damaged, or carries a different number than the paperwork shows, the steps are similar whether you are the driver or the receiver:
- Do not open the doors yet if you can avoid it.
- Photograph the seal, the doors, and the unit number.
- Notify the shipper, the carrier, and whoever arranged the shipment right away.
- Note the exact condition on the delivery receipt, including the seal number found.
- Count and inspect the freight carefully once the doors are opened, with photos throughout.
- Keep the seal itself if possible, since it can be examined later.
A seal discrepancy does not automatically mean theft. Paperwork errors and seals replaced during inspections are common causes. But it does mean the normal assumptions no longer hold, and careful documentation becomes essential. If goods are missing, our guide on cargo theft liability explains how responsibility is usually sorted out.
Delivery checks for receivers
Receivers should treat seal verification as part of the receiving process, not an optional step. Before cutting the seal, compare its number to the bill of lading or advance shipping notice, check that it is intact, and photograph it. Only then cut it and begin unloading. Write any discrepancy on the delivery receipt before signing. A clean signature on a receipt with a mismatched seal makes any later claim much harder to support.
Where technology helps
Seal numbers are a natural fit for automated checks. Document extraction tools can read seal numbers from bills of lading, and receiving apps can prompt dock staff to photograph and confirm the seal before unloading, flagging any mismatch automatically. Electronic seals and tracking devices add location and open-door alerts. These tools work best when they support a clear manual procedure rather than replace it.
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About the author: Luis Lopez is a Miami-based AI transportation consultant and logistics entrepreneur, the CEO of Go Hub.io Holdings Corp and subsidiaries, and host of the Freight Guru Podcast.
