The Containment relationship represents the containment of elements on the diagram pane. A Containment path replaces a Nesting representation. For example, instead of drawing one package nested into another, you can draw a containment relationship from one package to another (see the figure below). This example shows that the Containment relationship is a graphical representation as well as a nesting of elements. 
Example of package nesting (on the left) and package nesting using containment relationship (on the right) In the example above, the Containment end with a circle plus symbol shows a containing element. The User package is a contained element and the Domain package is a containing element: the owner of the User package is the Domain package. You can format containment symbol properties in the Symbol Properties dialog. | 