The SystemChangeNotification mechanism allows clients to receive notifications (events) when changes are made to the system. Such changes are additions, removals, renaming, ... of classes, events, instance variables, ... Eventually this system will be used as the foundation for the ChangeSet and the changes file, but for now these core entities were not yet changed (so it is quite safe to use, without the risk of damaging your system).
When you are interested in using the system, have a look at the class ChangeHooksTest (or other unit test classes) and/or browse the public protocol on the class SystemChangeNotifier.