You can promote a stereotype to metaclass status by selecting Use as Metaclass in the stereotype property page. Stereotypes promoted in this way have their own Browser folders and entries in the Model menu and New contextual menu.
Create new kinds of objects that share much of the behavior of an existing object type, such as business transactions and binary collaborations in a BPM for ebXML.
Have objects with identical names but different stereotypes in the same namespace (a metaclass stereotype creates a sub-namespace in the current metaclass).