All Service Plans come with one or more Application Databases. However, a Service Plan can connect to any database providing it has a Database Connection, for example, accessing a private database via a Gateway Tunnel.
Database Connections are configured in the Service Plan’s Admin Application and require a JDBC driver provided by the database manufacturer, which usually comes as a JAR file. A JDBC driver can be easily added to a Service Plan by adding it to the tomcatLib directory of the application's filesystem.
Application Databases automatically install a JDBC driver, so if you are connecting to another database of the same type, then there is no need to add its JDBC driver.