Extended objects, sub-objects, and links are special metaclasses that are designed to allow you to add completely new types of objects to your models, rather than basing them on existing PowerDesigner objects.
For more information on metaclasses, see Metaclasses (Profile). You should use extended objects, sub-objects, and links as follows:
Extended objects – can be created anywhere
Extended sub-objects – can only be created in the property sheet of their parent object where they are defined via an extended composition (see Extended Collections and Compositions (Profile))
Extended links – can be defined to link extended objects