Comment 2 for bug 551543

http://de.wikipedia.org/wiki/Proformarechnung (German)
http://en.wikipedia.org/wiki/Pro_forma
describes the various usages of the term "Pro Forma Invoice"

IMHO the content of a pro forma invoice depends largely on the usage and involved parties / countries

I think in most cases pro forma invoices must not produce account moves
- nevertheless, if the shipment of goods triggers VAT calculation AND no commercial invoice is issued in the same period - it might be useful to create the associated move lines.

One client reports that the pro forma invoice are used only for customs and are not posted in the accounting system.

a very complex issue.