Microsoft Exchange Server objects in the same class share a common set of attributes and behaviors. An object class derived from another class inherits all the attributes of that class. These derived classes, also called subclasses, usually include additional attributes of their own. For example, the classes Mailbox and Public-Folder are subclasses of the Mail-Recipient class, and both represent objects that can receive messages. In this case, the Mail-Recipient class is referred to as the superclass of Mailbox and Public-Folder. In the directory, any object class has exactly one superclass (except the Top class, which has no superclass).
Because objects inherit all attributes of the class above them, the schema can be represented using a tree structure. For example, the Mailbox-Agent class contains all attributes of the Mailbox class, the Mail-Recipient class, and the Top class because it inherits them from three levels of classes above it in the schema.