On the ops desk the SLI is the opening document of an export file: it gives the forwarder the exporter and consignee details, the cargo description, the shipping method and route, and any insurance or customs documentation requirements it needs to handle the shipment as the shipper intends. The decision it drives is whether the file is complete enough to act on, since a vague or incomplete SLI leaves the forwarder chasing the shipper for details before anything can move. It is not the same as the shipping instruction (SI), which goes to the carrier to drive the draft bill of lading; the SLI runs from shipper to forwarder at the start of the export, while the SI runs to the carrier later in the flow.
Glossary
Shipper's Letter of Instruction (SLI)
The shipper's written instructions to its freight forwarder for an export shipment, setting out the parties, the cargo details, the routing, and the insurance and documentation requirements, and authorising the forwarder to proceed with the shipment on those terms.