A scenario is a sequence of steps taken to accomplish a given goal. When the system is completed and the application is available, the customer should be able, in an easy and clearly specified manner, to accomplish all typical usage scenarios for the application.
The typical scenarios should represent the vast majority of uses for the system. The exact coverage of the system by the typical scenarios vary, but a 90 percent coverage is desirable. Obviously, a system with only one possible usage scenario will be easy to cover while a system with thousands of possible usage scenarios will be much harder to cover.
Frequently the 80/20 rule is invoked. Eighty percent of the functionality of a typical system is accomplished by twenty percent of the work. To accomplish the remaining minority functionality requires the vast majority of the work.