shipping-notifications/ShippingNotifications/etc/adminhtml/system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="cpcommerce_shipping_notifications" translate="label" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
            <class>separator-top</class>
            <label>Shipping Notification</label>
            <tab>cpcommerce</tab>
            <resource>Cpcommerce_ShippingNotifications::config</resource>
            <group id="general" translate="label" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                <field id="shipping_categories" translate="label" type="multiselect" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
                    <label>Shipping Categories</label>
                    <source_model>Cpcommerce\ShippingNotifications\Model\Config\Source\ShippingCategories</source_model>
                    <can_be_empty>1</can_be_empty>
                </field>
            </group>
        </section>
    </system>
</config>