In accounting you are told to only put VAT  Invoices onto the accounting system but Kashflow is the only system I know if where it allows PO for form part of the accounts. It gets very confusing which are purchase orders and what are invoices. Among other problems this allows you to pay a purchase order and if you receive an invoice the following month to when the PO is raised and change the dated to the invoice date so the VAT is in the correct month it will change your prior months accounts.