What a proforma invoice is for
A proforma invoice is a good-faith statement of what a transaction will involve, sent before it is finalised. It is not a demand for payment and is generally not entered into accounts receivable, which is precisely why it is useful: a buyer's finance team can raise a purchase order or release funds against it without it being booked as a liability.
It is also routinely requested for cross-border shipments, where customs authorities want a declared value for goods that have not yet been formally invoiced.
How it differs from a commercial invoice
A commercial invoice is the final, binding demand for payment and is recorded in your books. A proforma is provisional — the quantities, prices, or delivery terms may still change. For that reason, the document is titled clearly as a proforma and uses a "Valid Until" date rather than a payment due date.
Include a PO reference
Proforma invoices frequently travel through a buyer's procurement process, so the PO number field is available on this document type. Filling it in makes the document far easier for the recipient to match against their own paperwork.