A package in the Unified Modeling Language

Unified Modeling Language

Unified Modeling Language is a standardized general-purpose modeling language in the field of object-oriented software engineering. The standard is managed, and was created, by the Object Management Group...

 is used "to group elements, and to provide a namespace for the grouped elements". A package may contain other packages, thus providing for a hierarchical organization of packages.

Pretty much all UML elements can be grouped into packages. Thus, classes, objects, use case

Use case

In software engineering and systems engineering, a use case is a description of steps or actions between a user and a software system which leads the user towards something useful...

s, components

Component (UML)

A component in the Unified Modeling Language "represents a modular part of a system, that encapsulates its content and whose manifestation is replaceable within its environment. A component defines its behavior in terms of provided and required interfaces".A component may be replaced by another if...

, nodes

Node (UML)

A node In the Unified Modeling Language is a computational resource upon which UML artifacts may be deployed for execution.There are two types of nodes: device nodes and execution environments....

, node instances etc. can all be organized as packages, thus enabling a manageable organization of the myriad elements that a real-world UML model entails.

Usage

When organizing functional models (use case models, workflow models etc.), use packages to model the real-world modular structure of the system being modeled. When organizing source code, use packages to represent the different layers of the source code. For instance:

• presentation layer
  Presentation logic
  In software development, presentation logic is concerned with how business objects are displayed to users of the software, eg the choice between a pop-up screen and a drop-down menu...
  
  
• controller layer
• data access layer
  Data access layer
  A data access layer is a layer of a computer program which provides simplified access to data stored in persistent storage of some kind, such as an entity-relational database....
  
  
• integration layer
• business services layer

When organizing component models, use packages to group the components according to ownership and/or reuse possibilities. For instance:

• commercial-off-the-shelf products
  Commercial off-the-shelf
  In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...
  
  
• open-source framework components
• custom-built framework components
• custom-built application components

When organizing deployment models, use packages to represent the different types of deployment environments that you will be modeling. For instance:

• production environment
• pre-production environment
• integration test environment
• system test environment
• development environment
  Development environment
  In hosted software development, a development environment refers to a server tier designated to a specific stage in a release process....