app\models\Invoice::getAutoBillEnabled PHP Method

    public function getAutoBillEnabled()
    {
        if (!$this->is_recurring) {
            $recurInvoice = $this->recurring_invoice;
        } else {
            $recurInvoice = $this;
        }
        if (!$recurInvoice) {
            return false;
        }
        return $recurInvoice->auto_bill == AUTO_BILL_ALWAYS || $recurInvoice->auto_bill != AUTO_BILL_OFF && $recurInvoice->client_enable_auto_bill;
    }