What's the difference between the Shipment field on the Create Receipt page and the ShipmentNumber attribute in the Receiving Receipt Requests REST resource?

When receiving expected shipments, you can enter any value in the Shipment field on the Create Receipts page. This is a free text field.

The same isn't true about the ShipmentNumber attribute in the REST API. When the ShipmentNumber attribute is populated in the Receiving Receipt Requests REST resource, the value is treated as the ASN shipment number. There's no equivalent free text shipment field in the REST API.