magento-test/app/code/Magento/Sales/etc/extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Sales\Api\Data\OrderInterface">
        <attribute code="shipping_assignments" type="Magento\Sales\Api\Data\ShippingAssignmentInterface[]" />
    </extension_attributes>
    <extension_attributes for="Magento\Sales\Api\Data\OrderInterface">
        <attribute code="payment_additional_info" type="Magento\Payment\Api\Data\PaymentAdditionalInfoInterface[]" />
    </extension_attributes>
    <extension_attributes for="Magento\Sales\Api\Data\OrderPaymentInterface">
        <attribute code="notification_message" type="string" />
    </extension_attributes>
</config>