Every day, this automation finds every subscription that's due for renewal and processes the renewal charge for each one. On a successful charge, the customer gets the automated "Subscription: renewed" email. On a failed charge, the subscription is flagged so it shows as failing wherever you view subscriptions, the customer gets a renewal-failed email, and your store's admin email address also receives a separate "Subscription auto-renew error" notification so staff know a charge needs attention.