The premise behind an XP project metaphor is that all of the stakeholders on a project need to have a common, concrete means of talking about abstract concepts in the problem domain.

On the original Chrysler C3 payroll project in which XP arose, the team used the metaphor of an assembly line to discuss ways in which transactions of various kinds made their way through stages of payroll. An assembly line was something that everyone at Chrysler understood, and it helped them grok concepts in the payroll system that perhaps they did not all understand so readily.

Everyone should be referring to abstract problem-domain concepts with the same terms, to avoid confusion, and to maintain high production speed. An actual metaphor can be a rich source of terms used for everything from casual conversation to data structures in the code.

But actual metaphors can be hard to devise and enforce. Some people are more comfortable with metaphor than others. What is most important is that the team finds and shares a common vocabulary for the problem domain.

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