Zephyr classes and instances allow groups of people to have conversations via Zephyr. Zephyr classes are slightly more private than instances since you must know the name of a Zephyr class to be able to subscribe to it. Zephyr classes have sub-channels called instances. When someone talks about a Zephyr instance they usually mean an instance of the Zephyr class message, although all Zephyr classes have instances. Since class message is widely known, and it is possible to subscribe to class message, the instances of class message are not very private. Instances of class message are good for public discussions that are of interest to many people. For more private conversations amongst a group of friends a Zephyr class of some other name is probably preferable.
There are three parameters that control who will get a zephyrgram: a class, an instance, and a recipient. When you send a zephyrgram to a friend, using zwrite username, the class is message, the instance is personal, and the recipient is your friend's username. These parameters can be changed to allow conversaions amongst a group of people via Zephrgrams. What people call a Zephyr class refers to changing the class parameter to be something other than message. What people call a Zephyr instance, or public instance, refers to changing the instance parameter to something other than personal, while still within the class message. In the next few sections, we will explain how to change the class, instance, and recipient parameters to send and receive zephyrgrams to groups of people on different instances and classes.