Alistair Cockburn, in Writing Effective Use Cases, identified three levels of detail in writing use cases:
A brief use case consists of a few sentences summarizing the use case. It can be easily inserted in a spreadsheet cell, and allows the other columns in the spreadsheet to record priority, technical complexity, release number, and so on.
A casual use case consists of a few paragraphs of text, summarizing the use case.
A fully dressed use case is a formal document based on a detailed template with fields for various sections; and it is the most common understanding of the meaning of a use case. Fully dressed use cases are discussed in detail in the next section on use case templates.
Appropriate detail
Some software development processes do not require anything more than a simple use case to define requirements. However, some other development processes require detailed use cases to define requirements. The larger and more complex the project, the more likely that it will be necessary to use detailed use cases.
The level of detail in a use case often differs according to the progress of the project. The initial use cases may be brief, but as the development process unfolds the use cases become ever more detailed. This reflects the different requirements of the use case. Initially they need only be brief, because they are used to summarize the business requirement from the point of view of users. However, later in the process, software developers need far more specific and detailed guidance.
Rational Unified Process invites developers to write a brief use case description in the use case diagram, with a casual description as comments and a detailed description of the flow of events in a textual analysis. All those can usually be input into the use case tool (e.g., a UML Tool, SysML Tool), or can be written separately in a text editor.
