Study Cards on extension, goal, behavior
Can you name an extreme example?
When the extension purpose is not related to the core goal.
Consider the behavior “Fill Out Survey” when attached to the “Borrow Book” Use Case.
The “Fill Out Survey” is not necessary to complete the “Borrow Book” goal, even though we might attach them together, with the “Fill Out Survey” as an extension.
It may be a later version that allows the car to be reserved for a future time, “Reserve a Car.”
This “Reserve a Car” would be an extension of the normal path of “Hail a Ride.”
I do not believe that this is indeed legitimate if the extension is still required occasionally to meet the Base, however, you will see this in the UML literature
Many modelers would make “Pay Fine” an extension, though I would make it an Included Use Case because I believe it is needed to meet the goal of “Return Book.”
If you feel that Pay Fine is not required to implement Return Book, then make it an extension.
This distinction is tricky and it is probably not worth the time spent in arguing about this on a project.
It often winds up being a big source of contention that is not that significant
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