$connection->addColumn(
                $installer->getTable('quote_address'),
                'mob_type',
                [
                    'type' => MagentoFrameworkDBDdlTable ::TYPE_TEXT,
                    'nullable' => true,
                    'default' => NULL,
                    'length' => 255,
                    'comment' => 'Mob Type'
                ]
            );
        $connection->addColumn(
                $installer->getTable('sales_order_address'),
                'mob_type',
                [
                    'type' => MagentoFrameworkDBDdlTable ::TYPE_TEXT,
                    'nullable' => true,
                    'default' => NULL,
                    'length' => 255,
                    'comment' => 'Mob Type'
                ]
            );
        $installer->endSetup();

use MagentoCheckoutBlockCheckoutLayoutProcessor;

class MobPlugin
{
    public function afterProcess(LayoutProcessor $subject, $jsLayout) {
        $customAttributeCode = 'mob_type';
        $customField = [
            'component' => 'Magento_Ui/js/form/element/select',
            'config' => [
                'customScope' => 'shippingAddress.custom_attributes',
                'template' => 'ui/form/field',
                'elementTmpl' => 'ui/form/element/select',
                'id' => 'drop-down',
            ],
            'dataScope' => 'shippingAddress.custom_attributes.mob_type',
            'label' => 'Mob Type',
            'provider' => 'checkoutProvider',
            'visible' => true,
            'validation' => ['required-entry' => true],
            'sortOrder' => 150,
            'id' => 'drop-down',
            'options' => [
                [
                    'value' => 'local',
                    'label' => 'Local',
                ],
                [
                    'value' => 'vip',
                    'label' => 'VIP',
                ]
            ]
        ];

        $jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']['children']['shippingAddress']['children']['shipping-address-fieldset']['children'][$customAttributeCode] = $customField;

        return $jsLayout;
    }
}

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoCheckoutModelShippingInformationManagement">
        <plugin name="save_custom_field" type="NamespaceCustomModulePluginCheckoutSaveAddressInformation" />
    </type>

</config>

class SaveAddressInformation
{
    protected $quoteRepository;
    public function __construct(
        MagentoQuoteModelQuoteRepository $quoteRepository
    ) {
        $this->quoteRepository = $quoteRepository;
    }
    /**
     * @param MagentoCheckoutModelShippingInformationManagement $subject
     * @param $cartId
     * @param MagentoCheckoutApiDataShippingInformationInterface $addressInformation
     */
    public function beforeSaveAddressInformation(
        MagentoCheckoutModelShippingInformationManagement $subject,
        $cartId,
        MagentoCheckoutApiDataShippingInformationInterface $addressInformation
    ) {
        $shippingAddress = $addressInformation->getShippingAddress();
    $shippingAddressExtensionAttributes = $shippingAddress->getExtensionAttributes();
    if ($shippingAddressExtensionAttributes) {
        $customField = $shippingAddressExtensionAttributes->getMobType();
        $shippingAddress->setMobType($customField);
    }

    }
}