As a manufacturer, when I receive a pro forma payment, I am not going to be turning that pro forma into an invoice for at least two weeks. I have to allocate the payment on account and enter reference details to keep track of which job the payment is for. I can have many "payment on account" in a single account. This, essentially, makes the pro forma feature useless. There's no point in me sending a pro forma - it's just extra work. If, however, the pro forma invoices were uniquely numbered, and I could record a payment against a proforma - all of the referencing legwork would be gone. Each payment for each job would be tracked along the way. Then, when I send out the product, I can turn the pro forma into an invoice and the payment allocations will already be listed on the invoice.