The term Advance Shipping Notice (ASN) refers to an electronic document that contains detailed information about an upcoming shipment.
Backorder = Backorder
The respective item is currently not in stock.
Option 1: Ship the number of items already in stock and send them back with a second order as soon as the rest arrives in the warehouse.
The second possibility is to hold the goods back until everything the customer has ordered is in stock and can be sent in a consignment.