TL;DRThe entity that receives the invoice and is expected to pay it.
In subscription mode the bill-to party is the customer on the schedule. In shipment mode it is the shipper — the company that booked the haul — not the carrier and not a generic customer record.
Collections, bank matching, and revenue all use the bill-to party as the counterparty, so who gets chased and whose deposit is matched is decided once, on the document.
Where it happens in Sonic
Next
See it on your contracts
A walkthrough on the agreements and files you actually bill from — not a slide deck.