"When an expense note of a user was confirmed, previous versions required a supplier to be defined for the expense amount to be paid out. Now, OpenERP v7 uses vouchers to handle this instead of invoices." Documentation from v7.

However, to manage taxes, we need to generate an invoice because in a voucher you have only one tax for all the lines; but in an invoice, you could have one tax PER line. And we need to manage one tax per line according the product used in the expense note line.