The BestEffort mode means that the application is notified about all the resultset updates, but may also produce false positive notifications. The Guaranteed mode means that notification are sent only if the query result data were indeed changed in the database. Note that the Guaranteed mode has stricter requirements for a query to register, and some queries that can be registered for notifications in the BestEffort mode, cannot be registered in the Guaranteed mode. See more details about requirements to queries in Oracle documentation.