Comment 4 for bug 1155606

yes there exists use case where the tax amount is different than the debit or credit
* in Belgium, because we may have taxes that impact several (more than one) tax codes (for base amount or for tax amount), so that it ends in openerp with a line with debit=credit=0 and a tax amount. (and it's not dedicated to Belgium: if i remind correctly in Luxembourg they have that kind of taxes too)
* as i said, if your tax is a fixed amount (not a percentage) (e.g taxe Récupel)