I'm just Confused with this:

amount_invoice.py:

 def _amount_residual(self, cr, uid, ids, name, args, context=None):
        result = {}
        for invoice in self.browse(cr, uid, ids, context=context):
            result[invoice.id] = 0.0
            if invoice.move_id:
                for m in invoice.move_id.line_id:
                    if m.account_id.type in ('receivable','payable'):
                        result[invoice.id] += m.amount_residual_currency
            #prevent the residual amount on the invoice to be less than 0
            result[invoice.id] = max(result[invoice.id], 0.0)            
        return result

account_move_line.py:

def _amount_residual(self, cr, uid, ids, field_names, args, context=None):
        """
           This function returns the residual amount on a receivable or payable account.move.line.
           By default, it returns an amount in the currency of this journal entry (maybe different
           of the company currency), but if you pass 'residual_in_company_currency' = True in the
           context then the returned amount will be in company currency.
        """
        res = {}
        if context is None:
            context = {}
        cur_obj = self.pool.get('res.currency')
        for move_line in self.browse(cr, uid, ids, context=context):
            res[move_line.id] = {
                'amount_residual': 0.0,
                'amount_residual_currency': 0.0,
            }

            if move_line.reconcile_id:
                continue
            if not move_line.account_id.type in ('payable', 'receivable'):
                #this function does not suport to be used on move lines not related to payable or receivable accounts
                continue

            if move_line.currency_id:
                move_line_total = move_line.amount_currency
                sign = move_line.amount_currency < 0 and -1 or 1
            else:
                move_line_total = move_line.debit - move_line.credit
                sign = (move_line.debit - move_line.credit) < 0 and -1 or 1
            line_total_in_company_currency =  move_line.debit - move_line.credit
            context_unreconciled = context.copy()
            if move_line.reconcile_partial_id:
                for payment_line in move_line.reconcile_partial_id.line_partial_ids:
                    if payment_line.id == move_line.id:
                        continue
                    if payment_line.currency_id and move_line.currency_id and payment_line.currency_id.id == move_line.currency_id.id:
                            move_line_total += payment_line.amount_currency
                    else:
                        if move_line.currency_id:
                            context_unreconciled.update({'date': payment_line.date})
                            amount_in_foreign_currency = cur_obj.compute(cr, uid, move_line.company_id.currency_id.id, move_line.currency_id.id, (payment_line.debit - payment_line.credit), round=False, context=context_unreconciled)
                            move_line_total += amount_in_foreign_currency
                        else:
                            move_line_total += (payment_line.debit - payment_line.credit)
                    line_total_in_company_currency += (payment_line.debit - payment_line.credit)

            result = move_line_total
            res[move_line.id]['amount_residual_currency'] =  sign * (move_line.currency_id and self.pool.get('res.currency').round(cr, uid, move_line.currency_id, result) or result)
            netsvc.Logger().notifyChannel("111111111111.",netsvc.LOG_INFO, '' +'' + str(res[move_line.id]['amount_residual_currency']))
            res[move_line.id]['amount_residual'] = sign * line_total_in_company_currency
            netsvc.Logger().notifyChannel("222222.",netsvc.LOG_INFO, '' +'' + str(res[move_line.id]['amount_residual']))
        return res