Hi Steve,

From the Adagio Receivables help file notes for Import requirements:

Hdr-Document Number is not required unless “Header fields on each record?” is selected. However, if the option is off and Hdr-Document number is not imported, the Batch and Entry number will be assigned as the document number, in a format 'B-E'.

AR standalone does not support an incrementing invoice number, but instead defaults a unique value derived from the batch-entry numbers.
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From the Adagio Invoices help file notes for Import requirements:

If the Hdr-Invoice no is null in the import file (no characters nor spaces) and the ‘Increment’ option is selected in the Company Profile, then the next document number will be assigned. If Increment is not on, the imported invoice number will be the value in the same next document number field, but the same for all imported invoices.

If you don't import the invoice number field (or do import it but the import file value is empty), then importing will assign a sequentially incremented invoice number, provided that is how you have set up the Company Profile in Adagio Invoices for document numbering.