Question

What a "base” use case is?

DQ 7-1 OOA

Answer 1

UML Use Case

Use cases allow to capture requirements of systems under design or consideration, describe functionality provided by those systems, and determine the requirements the systems pose on their environment.

Assuming use case could be abstract and applying appropriate definition for the classifier, abstract use case is use case which does not have complete declaration (is incomplete) and ("typically", as UML specification says) can not be instantiated. An abstract use case is intended to be used by other use cases, e.g., as a target of generalization relationship.

Web User Authentication use case is abstract use case
specialized by Login, Remember Me and Single Sign-On use cases.

Though UML specification avoids doing it, it is quite common to find sources that define including use cases as abstract use cases or essential use cases. While we may assume that including use cases are always abstract, included use case could probably be either abstract or concrete. Amazingly, there are some sources - that I can't agree with - providing exactly opposite explanation that including (base) use cases are "usually concrete", while included ("addition") use cases are "usually abstract".