Architecting and designing : UML 2.0 Lite : UML models : Introductory tour of UML diagrams : Developing systems with UML : Organizing the system by packages
  
Organizing the system by packages
UML introduces the notion of a package as the universal item to subdivide and categorize systems. Packages are used on every level, from the highest level, where you are subdividing the system into domains, to the lowest level, where you may be subdividing your class diagram into groups of classes each belonging to a package.
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Business analysis with Use Cases
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Developing systems with UML