Metaclasses are classes drawn from the PowerDesigner metamodel, and appear at the top level of the Profile category. You add a metaclass to a profile when you want to extend it.
Concrete metaclasses are defined for specific object types that can be created in a model, while abstract metaclasses are never instantiated but are instead used to define common extensions. For example BasePackage is an ancestor to both model and package.
For information about viewing and navigating among metaclasses in the metamodel, see The PowerDesigner Public Metamodel.
If you do not want to extend an existing metaclass, but rather create an entirely new kind of modeling object, you should use the extended object metaclass (see Extended Objects, Sub-Objects, and Links (Profile)).