If you choose to save your invoice as a draft, it will be saved under ‘Draft invoices’. Use the function if you want to create an invoice and send it at a later date or to prepare drafts of several invoices. Be aware that the billing date becomes the date you created the invoice unless you specifically choose otherwise. If you send the invoice at a later date the billing date will not change.
Your draft won't receive an invoice number until you choose to send it as an invoice, to be able to keep the invoice numbers in the correct order.