Reference cited from Alistair Cockburn's Gold nuggets found in his book Writing Effective Use Cases .
Cockburn points out, "We will not actually write every scenario separately from top to bottom. That is a poor strategy because it is tedious, redundant, and hard to maintain."
A use case can be written nonredundantly, starting with an unconditional main flow that is subsequently enhanced with conditional additional flows of different kinds, where a flow is a sequence of steps.
The main flow equates to the main success scenario and each additional flow represents a portion of one or more of the remaining scenarios.
The UML does not deal with flows. Cockburn only refers to the main success scenario (main flow) and extensions (additional flows). Refining this view, the following sections present five different kinds of flow by providing a definition, diagram and simplified example for each.
The purpose of the examples is to illustrate a clear, consistent and scalable writing convention for representing a use case's structure (they are not meant as full-fledged use case specifications).
Step through the process of adding use cases to the use case diagram: