In any system, the most important thing is whether it's doing what it's supposed to do for all of the use cases that it's supposed to support. Code has to be written for each of those use cases, and each use case corresponds to one or more business processes or rules, so it's only logical that each of those use cases needs to be defined and documented to whatever extent is appropriate for the development process. As with the logical and physical architecture, it's possible to execute those definitions as either text or some sort of diagram, and those approaches have the same advantages and drawbacks that were noted before.
The Unified Modeling Language (UML) provides a high-level diagramming standard for use cases, useful mostly for capturing the relationship between specific types of users (actors, in UML&apos...