The Extended Dependencies page in property sheets allows you to define your own semantic dependencies between objects of the model. They are not interpreted and checked by PowerDesigner. These dependencies can be created to document a special relationship between objects, they are meant to be used for documentation purposes only.
Each time an extended dependency is defined on the dependent object, the influent object displays the inverse relationship in the Extended Influences tabbed page of the Dependencies page.
You could create an extended dependency between an actor and a file to attach a .RTF file to an actor. It will describe the actor in a .RTF format instead of using the Annotation tabbed page in the Notes page. In this case, you can use a <<documentation>> stereotype in the Extended Dependencies page of the actor property sheet.
For more information on Extended Dependencies, see "Using extended dependencies", in the Objects chapter of the Core Features Guide.