scope:

Watchers are defined as entities that request (i.e., subscribe to) information about a resource. There is fairly complex state associated with these subscriptions. The union of the state for all subscriptions to a particular resource is called the watcher information for that resource. This state is dynamic, changing as subscribers come and go. As a result, it is possible, and indeed useful, to subscribe to the watcher information for a particular resource. In order to enable this, a format is needed to describe the state of watchers on a resource. This specification describes an Extensible Markup Language (XML) document format for such state.