A Deployment diagram Package diagram falls under the structural diagramming family. A Package diagram shows the structure of the system at the level of Packages. Generally, it arranges and organizes packages and Dependencies between the Packages. Package is the major element used in the Package diagram.
Package. A Package diagram uses Packages to group UML elements. They are also used to provide a namespace for the grouped elements. Just like classes can be grouped into packages, packages can be nested within other packages. A Package, as an entity, may have all the relationships that can be drawn for a class. Those relationships are derived from the classes or packages that are nested within two particular packages (i.e., the relationship between packages reflects a set of relationships between classes placed in those packages).
Purpose
A Package diagram shows the arrangement and organization of model elements in a project. It shows both structure and dependencies between sub-systems or modules.
Usage
A Package diagram can be used to:
organize large systems containing diagrams, documents, and other deliverables
simplify complex Class diagrams.
Summary
Package diagrams are valuable because they:
can be used to structure high level model elements
can be used to create an overview of a large set of models
