10+ Use Case Extension Point Example. Consider a scenario where a user is buying an item from an online shopping site. Also, note that an alternate flow in one use case could also contain an extension point which references another extending use case.
Although your problem or scenario won't be exactly like this you can use them as a starting point. The constructor is called with the parameters that are passed to the perform class method. The extensions section defines alternative paths through the use case.
Also here's the definition of extension point from the uml reference manual (2nd edition) (p358)
10+ Use Case Extension Point Example. Suppose a person generates a support request on a website for electronic products. The extending use case accomplishes this by conceptually inserting additional action sequences the red text in figure 2 presents an example of how an extension point would be indicated in the if several use cases extended this one from the same point, then each one would need to be listed. For example, other use cases may be inserted into the project selected extension point just described for the manage project use case. For example, in step 3 of the create account use case, username is already in use and email.