A containment line is used to show, for example, parent-child relationships between two classes or two packages.
To illustrate containment between two classes:
1.Click the Containment toolbar button (in a class or package diagram).
2.Drag from the class that is to be "contained", and drop on the container class.
Note that the contained class, Engine in this case, is now visible in a compartment of Car. This also places the contained class in the same namespace as the container class.