In Application Server 7.x and 8.x, most of the Application Server core classes were loaded by the by the System Classloader, which also loaded Java Virtual Machine (JVM) classes. In Application Server 9, the System Classloader loads JVM classes but most of the Application Server core classes have been moved to the new Shared Chain Classloader.
In the Migration Tool, on the JVM Settings page, under the Configuration component, there is a new field called System Classpath. This contains the JVM classes. Classes cannot be deleted from this field, but they can be added. The Server Classpath field no longer shows the Application Server core classes, and its use is discouraged. Use Classpath suffix instead.