With respect to the zephyr messaging system, a class functions as a cross between a BBS and a chatroom. Each zephyr has a class and an instance. Standard personal zephers are class "message", instance "personal", but a user can send a message to any class or instance.
To get messages sent to a class, a user subscribes to that class. Users may subscribe to all instances of a class by using a wildcard, or to specific instances. Users may not, however, use a wildcard to subscribe to all classes. This allows the existence of private zephyr classes, with unpublished names.
Certain zephyr classes and instances are well-known and high traffic, such as MIT's class help. On many such public classes, it is conventional and good manners to use the instance of your zephyr to denote your topic, to enable many parallel conversations.