“Various styles for writing use cases are presented with examples and discussions of their relative advantages and disadvantages, particularly their consequences for user interface design and software usability. Essential use cases, a variant employed within usage-centered design, are contrasted with conventional use cases and scenarios. For the most efficient support of user interface design and particularly for large, complex projects, a highly-structured form of use case has evolved. New narrative elements and relationships among use cases are introduced. These include means for expressing partial or flexible ordering of interaction, relationships with business rules, as well as a clarification of the often misunderstood concept of extension that recognizes two distinct forms: synchronous and asynchronous extensions.” (Constantine & Lockwood, Ltd.) – courtesy of guuui