Each time an invoice is created it will appear in the accounts report whether it has been paid or not. For VAT registered organisations, this means when you create an invoice, a VAT liability occurs at the point of creation. Some organisations therefore choose to create a pro-forma invoice and only once the full funds have been received is the full invoice created to bring the income into their account.

For all organisations, a proforma invoice can be a useful option to request payment; particularly where a trader or sponsor may not be guaranteed to confirm their booking or they may change their mind on which product they are purchasing. If it goes unpaid it will not affect your accounts and if the trader changes their product you will not need to cancel the invoice and raise a credit note before making the necessary amendments. 

Over a period of time, it all balances out providing that invoices are cancelled that don't ever get paid.

It may be worth taking advice from your accountant as to which process best suits your operation.