Levels of Use Cases

A common problem with use cases is that by focusing on the interaction between a user and the system, you can neglect situations in which a change to a business process may be the best way to deal with the problem. Often, you hear people talk about system use cases and business use cases. The terms are not precise, but in general, a system use case is an interaction with the software, whereas a business use case discusses how a business responds to a customer or an event.

[Cockburn, use cases] suggests a scheme of levels of use cases. The core use cases are at "sea level." Sea-level use cases typically represent a discrete interaction between a primary actor and the system. Such use cases will deliver something of value to the primary actor and usually take from a couple of minutes to half an hour for the primary actor to complete. Use cases that are there only because they are included by sea-level use cases are fish level. Higher, kite-level use cases show how the sea-level use cases fit into wider business interactions. Kite-level use cases are usually business use cases, whereas sea and fish levels are system use cases. You should have most of your use cases at the sea level. I prefer to indicate the level at the top of the use case, as in Figure 9.1.