Recurring script can be used to run some script that need to executed whenever we issue php bin/magento setup:upgrade irrespective of version change.

The recurring script will be run after the module setup script every time the command line

php bin/magento setup:upgrade

This script will be defined same as InstallSchema class but difference in name of the class. The example for this class you can see in vendor/magento/module-indexer/Setup/Recurring.php