If a transaction involves multiple Ship-From locations, you can specify the locations at the line level so that tax can be accurately calculated for each line.
You must have the Multi-Location Inventory feature turned on in NetSuite SuiteTax. You can read more about how to do this in the NetSuite Help Center.
When adding an item line to a transaction, you can set the ShipFrom location for that item in the Location column.
If you don't set an item line location, NetSuite will use the document-level location, which you can set in the Primary Information section.