Automatic Billing Updater (ABU) is Mastercard's branded version of a card account updater, the service that tells merchants when a customer's stored card has been replaced, renewed or closed, and that entry explains the service in full.
When an issuer sends a new card with a different number or expiry date, it reports the change to the network. Merchants that keep cards on file, such as subscription services, utilities and online retailers, or their acquirers, send their stored card list for checking or receive updates automatically. They then replace old details before the next charge, so that recurring payments and card-on-file transactions are not declined. The service can also report that an account has been closed or that the cardholder has asked the merchant to be contacted.
Visa offers a similar service, Visa Account Updater, and other networks have their own versions; "card account updater" is the generic term covering all of them. Updaters reduce involuntary churn and failed payments, and work alongside network tokenization, which can keep a stored credential valid even when the underlying card number changes.