Use case relationships fall into the category of things that "seemed like a good idea at the
time". I suggest that you actively ignore them. They’ll add no value to your use cases, or
to your understanding of the system, and they will be the source of many never ending
debates about whether or not to use «extends» or «generalization».

I tend to agree...

My understanding:

A use case "UCA" can "extend" another, "UCB", without the other one's "knowing" about the first one. To do this, UCB defines "extension points", referring to spots in UCA, where there should be a new branch to new activities defined in UCB, followed by a return to the original sequence of activities in UCA, at the extension point. From the perspective of UCA, nothing unusual has occurred, but actually at the extension point, activities of UCB were "inserted".