To start using Pro Forma invoices in XTRF, you need to enable them first in Configuration > → Clients and providers > → Client settings > → Payment terms tab > → tick use pro forma invoice checkbox:
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Once it’s it is enabled, in the Invoicing module > → Add task-based client pro-forma invoice button will be enabled:
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The Pro Forma invoices have an individual numbering scheme (Configuration menu → Templates → Numbering Schemes).)