The use case is a method of capturing software requirements. A user-system interaction sequence is used in a use case specification to explain the System's functionality. The main flow of the events describes the System's single path, and alternative flows and exception flow represent different routes through the System. Unlike the standard flow, an alternative flow depicts a situation that enables users to achieve their objectives. It's commonly thought of as an optional flow, suggesting that the user has selected to follow a different route through the System. An exception flow is an unintentional course that the System takes, frequently due to missing data or problems with the System's availability. For the user, exception flows signify an unwanted route.