This approach is to send a vendor bill initialize request that references one or more purchase orders by id. The initializeResponse returns itemList and expenseList data from the referenced purchase order(s). This data includes orderDoc and orderLine field values that together provide a unique identifier for each line item. You can then send a vendor bill add request setting values for the vendor bill’s itemList and expenseList with data copied from the response.
You should copy only the data from the specific purchase order items and expenses that you want to be included in the vendor bill. This approach lets you select a subset of line items from each linked purchase order, rather than including all linked purchase order line items in a vendor bill.
In this approach, you will use the initialize operation, which supports multiple purchase orders per vendor. Also, the orderDoc and orderLine fields are included in the vendor bill itemList and expenseList.
For more details: NetSuite Applications Suite – Purchase Order