A use case specification describes the functionality of a system in terms of a sequence of user-system interactions.  The main flow of events describes a single path through the system.  It represents the most common way that the use case plays out successfully and contains the most popular sequence of user-system interactions.  Other scenarios or paths through the system are described in alternative flows and exception flows.  So what is the difference?

First, it’s worth saying that there are a number of opinions in this area since the Unified Modeling Language has no standard for Use Case Specifications.  Some authors mention only alternative flows and use them for both optional flows and error flows.  However, of those authors that do differentiate between alternative flows and exception flows some agreement in definition has emerged. 

An alternate flow describes a scenario other than the basic flow that results in a user completing his or her goal.  It is often considered to be an optional flow and implies that the user has chosen to take an alternative path through the system.  An exception flow is an unintended path through the system usually as a result of missing information or system availability problems.  Exception flows represent an undesirable path to the user.  However, even though the exception flow has occurred the system  will ideally react in a way that recovers the flow and provide some useful information to the user.

The primary benefit of differentiating between alternative flows and exception flows is the focus that exception flows bring to error conditions.  By capturing all of the ways that the system can fail or produce an error, the business analyst can be sure to create a design which mitigates the impact of the error.