Displays a list of categories for all event source types, or, if specified, for a specified source type. You can also see this list in the "Amazon RDS event categories and event messages" section of the <a href="https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Events.Messages.html"> <i>Amazon RDS User Guide</i> </a> or the <a href="https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/USER_Events.Messages.html"> <i>Amazon Aurora User Guide</i> </a>.