Use-case-generalizations are used to show that workflows share structure, purpose, and behaviors. A parent use case may be specialized into one or more child use cases that represent more specific forms of the parent. This is generalization as applicable to use cases.
Once you have outlined the workflow of each business use case, you will find structures and behavior that is common to several business use cases. To avoid describing the same workflow several times, you can put the common behavior in a business use case of its own.
A use-case instance executing a child use case will follow the flow of events described for the parent use case, inserting additional behavior and modifying behavior as defined in the flow of events of the child use case.
You should reconsider models that have more than one level of use-case-generalizations. Layers of this kind make models hard to understand, even if they are correct in all other aspects.
Rational Unified Process