A pro forma invoice is a provisional invoice or a draft that has no effect at the accounting level. Therefore, it is not recorded in the accounting of a company.
It is usually used for information purposes so that the client has all the details as a prior step to the sales invoice.
As it is not an official document, it has no obligations regarding the information that must appear, but it usually includes the same fields as the invoice.
It is not mandatory to use pro forma invoices, it is the choice of each business to use this type of document.