Live data source - External shortcuts are created for all selected objects that already exist in another target model. These existing objects are deselected by default in the Selection tab of the Reverse Engineering dialog box, except for target objects corresponding to shortcuts already existing in the reversed model. 

Shortcuts and object replications allow you to reuse objects defined in one model or package elsewhere. You should use a shortcut when you want to reference an object in another model or package, and a replica when you want to create a local copy of the object and make changes to one or more of its properties.

Reusability - You can create libraries of reusable objects whose properties are inherited by multiple objects in different models. For example, the Person entity and its four attributes (Name, Age, Nationality and Address) can be used via inheritance links in any model where you have need of a customer, employee, etc.

Live data source - External shortcuts are created for all selected objects that already exist in another  target  model. 

Shortcuts and replicas that you create yourself, as well as the shortcuts that PowerDesigner creates automatically when you link or map objects or perform other tasks, all point to objects in other models, which are called target models. 

These models are listed in the Browser in the Target Models category with status icons to indicate whether your local copies are the latest available in the repository (see The Browser). 

This list of models is also available by selecting Model > Target Models.

You can use the tools on this list to open a target model, view its properties, or change or delete it. For information about all the tools available on this list, see Object Lists.

Note: If you delete a target model from the List of Target Models, you will also delete all the shortcuts that reference it in the present model and transform any replicas that reference it into independent objects. You cannot undo deleting or changing a target model