To answer that, I need to introduce another Use Case term: the PATH.
The steps you documented represent a specific PATH through the Use Case.
They were the exact actions and reactions that you experienced in executing the assignment.
If you had to create a different Use Case every time something was different (e.g., no power), you would end up with a thundering herd of Use Cases and a lot of the steps would be identical, meaning redundant.
To avoid redundancy, the Use Case paradigm differentiates between a “Standard Path” (aka Basic Course of Events – BCOE or Happy Path – which we do not recommend as it implies an emotional component), and “Alternate Paths” (i.e., the path less taken).
The Standard Path represents the interactions under “normal” circumstances, meaning nothing ever goes wrong (which was, by the way, the rationale for calling this the “happy” path).
Generally speaking, there will only be one standard path in a Use Case.
It can have any number of Alternate Paths, one for each time you want to document how to deal with a specific situation.
How do you represent this in the scenario we just created? There are several possible solutions and we recommend using the “AT” convention.