Unique Elements are those plan elements that may occur on multiple plans of one project and that preserve their identity across plans. The counterpart to a Unique Element is a Local Element, an element whose scope is limited to one particular plan.
Unique Elements are displayed in a dedicated view, the Repository View. This view displays all Unique Elements of a project, as well as, their hierarchical de-composition relationships.
When you select a Unique Element in the Repository View, its occurrences will automatically be highlighted in the currently open Editor.
The programs tracks occurrences of all Unique Elements. When you select a Unique Element in the Repository or in the Editor, you get an overview of all plans with
occurrences of the selected Unique
Element in the Details View.
Those plans in which the Unique Element
is refined, carry a small green arrow 
in their icon.
You can open any of the plans via double click.
Alternatively, you can directly double-click a Unique Element in the editor, which launches a dialog that shows all plan in which the Unique Element is refined.
Related Topics:
Creating a Unique Element
Editing a Unique Element
Editing multiple Unique Elements
Deleting a Unique Element
Adding a Unique Element Occurence
Removing a Unique Element Occurence