For a bill in Quickbooks we have a field called Ref No. I thought that it might populate it with the document reference value.
I understand that this only works if an invoice number is picked up automatically.
In the case of a receipt - with no clear document number, the entry for document reference is empty.
Therefore we are manually entering a reference number. In the case where we have used a manually entry for the document reference number, and approve to send to quickbooks, the Ref No. field remains unpopulated.

The current behaviour was described by tech support as the following:

The reference number field in QBD will be populated if there's an Invoice Number extracted from the image in Dext Prepare. In cases where you don't have an Invoice Number, you can use the Description field instead, which will be transferred to QBD in the Memo field