Proformas are used for formal quotes, advance payments or customs procedures. If you collect an advance, you must issue an invoice for that payment at that moment, VAT included; it also helps to label the proforma clearly as such and keep it outside the numbering of your invoice series, so nobody mistakes it for a real invoice.
A common example: a foreign client asks you for a proforma so their bank will approve the transfer or to handle the import paperwork. You send the proforma, they pay, and at that moment you issue the final invoice with its series number and record it. The proforma stays as a supporting document, with no further effect.
Since it is not an invoice, the proforma does not go into the record book of issued invoices or the VAT return, and it does not generate a registro de facturación under Verifactu. What counts is the invoice you issue afterwards, or the advance-payment invoice, which is booked with its entry and reported in the right quarter.
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