U.S. Customs and Border Protection (CBP) sends the 355 transaction set to importers, brokers, and freight forwarders to communicate acceptance or rejection decisions on customs manifests and related cargo documentation. This message typically responds to manifest submissions and provides detailed feedback on manifest entries, amendments, and in-bond shipments. The M10 segment conveys core manifest identifying information, the P4 segment specifies U.S. port details relevant to the decision, and the K1 segment (used throughout) allows CBP to attach remarks clarifying the acceptance or rejection rationale at various detail levels.

For example, when United States Customs Brokers, Inc. submits an ocean manifest covering containerized goods arriving at the Port of Los Angeles, CBP reviews the bill of lading data, commodity descriptions, and shipper details. If CBP accepts the manifest with minor corrections, it returns a 355 transaction set to USCB with the M10 segment restating the manifest reference number, a P4 segment confirming Los Angeles as the entry port, and K1 remarks noting that three commodity line items required clarification but were resolved. Alternatively, if CBP rejects specific in-bond movements, the 355 includes M12 segments (in-bond identifying information) with associated remarks explaining noncompliance, allowing USCB to correct and resubmit before the goods can be released or transferred.