When you create a new purchase invoice you will add parts to it.  Unlike Sales Invoices you can add negative and positive valued items and Onyx will decide whether the final document is a sales invoice or credit note when it saves the document.  If the overall total of the document is positive then Onyx will call the document a Purchase Invoice, if the overall total is negative then Onyx will call the document a Purchase Credit Note.

Whilst you can mix and match positive and negative items within a purchase invoice, if you plan to export to Sage you should not as Sage doesn't support this.