Magento 2 system.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../Magento/Config/etc/system_file.xsd">
    <system>
        <tab id="tab_id" translate="label" sortOrder="10">
            <label>Some Tag Label</label>
        </tab>

        <section id="section_id" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="0" showInStore="1">
            <label>Some Section Label</label>
            <tab>tab_code</tab>
            <resource>Some_resource::some_resource</resource>

            <group id="category" translate="label comment" type="text" sortOrder="20" showInDefault="1" showInWebsite="0" showInStore="0">

                <field id="root_category_id" translate="label comment" type="select" sortOrder="20" showInDefault="1" showInWebsite="0" showInStore="0">
                    <label>Default Root Category</label>
                    <source_model>VENDOR\EXTENSION_NAME\Model\Config\Source\Category</source_model>
                    <validate>required-entry</validate>
                    <comment>Some comment.</comment>
                </field>

            </group>

        </section>
    </system>
</config>