If all the inventory for your PO is not arriving at the same time, you must create an ASN for each shipment.
Use the same customer_purchase_order_id (your PO number) on the files so you can easily reference all shipments for your PO.
For the customer_identifier (your ASN name), use the same ID and add a suffix of “^” plus an incrementing number (e.g. 2928100, 2928100^1, 2928100^2) so you can easily find all ASNs for the same PO.