UML 2 Tutorial - Package Diagram
Package diagrams are used to reflect the organization of packages and their elements. When used to represent class elements, package diagrams provide a visualization of the namespaces. The most common use for package diagrams is to organize use case diagrams and class diagrams, although the use of package diagrams is not limited to these UML elements.
The following is an example of a package diagram.
Elements contained in a package share the same namespace. Therefore, the elements contained in a specific namespace must have unique names.
Packages can be built to represent either physical or logical relationships. When choosing to include classes in specific packages, it is useful to assign the classes with the same inheritance hierarchy to the same package. There is also a strong argument for including classes that are related via composition, and classes that collaborate with them, in the same package.
Packages are represented in UML 2.1 as folders and contain the elements that share a namespace; all elements within a package must be identifiable, and so have a unique name or type. The package must show the package name and can optionally show the elements within the package in extra compartments.
A «merge» connector between two packages defines an implicit generalization between elements in the source package, and elements with the same name in the target package. The source element definitions are expanded to include the element definitions contained in the target. The target element definitions are unaffected, as are the definitions of source package elements that don't match names with any element in the target package.
The «import» connector indicates that the elements within the target package, which in this example is a single class, use unqualified names when being referred to from the source package. The source package's namespace gains access to the target classes; the target's namespace is not affected.
The nesting connector between the target package and source packages shows that the source package is fully contained in the target package.