CO
Ocean freightContainer Operator
The shipping line or lessor responsible for a container, as recorded in port and depot systems.
In practice
Terminal systems like PORTNET tag every box with its container operator code so that gate moves, storage, and empty returns are billed to the right party. You will see it on depot receipts and EDI messages rather than in customer correspondence.
Seen in the wild
Gate-in rejected: CO code on the booking does not match the box prefix.