TL;DRThe company that booked the freight and receives the invoice for it.
In shipment mode the shipper is the bill-to party, kept in its own catalog with aliases for the many ways a name appears on upload files. The carrier is the hauler and sits in a separate catalog with its contracts.
Upload processing matches each row's shipper name to the catalog, suggests near matches, and lets the operator create or link before any shipment is created. Rows with no linked shipper are not persisted.
Where it happens in Sonic
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See it on your contracts
A walkthrough on the agreements and files you actually bill from — not a slide deck.