I'm trying to set the items for an inbound shipment, the first column is a select drop-down. Via script is that p.o. number or internal id /string /int? also setting the value for the item is it the internal id or lineuniquekey

inboundShipment.setSublistValue({
        sublistId: "items",
        fieldId: "purchaseorder",
        value: // p.o.  number  or internal id /string /int ?,
        line: i,
      });
      inboundShipment.setCurrentSublistValue({
        sublistId: "items",
        fieldId: "shipmentitem",
        value: lineuniquekey,
        line: i,
      });