The scenario I want to describe branches into several possible outcomes, or has some error conditions. How can I represent that with Use Case Diagrams?

Representing failure and branching conditions is often best done with a Sequence Diagram or flow chart, but there are some grey-area cases when it isn't clear whether or not a Use Case Diagram is appropriate. A rule of thumb: if in representing the branching actions in the Use Case Diagram you must add significantly more use case ovals, and the resulting diagram is muddy or confusing, consider using a different diagramming style. With that said, it is possible to represent simple branching behavior with UCDs, although I would like to again stress that UCDs ARE NOT FLOW CHARTS.

This is done by realizing that if the use case, or process that you are trying to represent can have two significantly different outcomes (success and failure, for example), then that means that you really have two different use cases: one in which the process succeeds, and one in which the process fails. Of course, these two use cases are related in that they are both extensions of the original use case, so ... more.

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