What are the two major types of relationships that can exist among use cases?

The two major types of relationships that can exist among use cases are indeed extend and include.

The "include" relationship is used when a use case incorporates the functionality of another use case. This establishes a dependency where the included use case provides specific behavior that is reused by the base use case. For example, if there is a primary use case of "Process Order," a use case such as "Validate Payment" could be included, indicating that validating the payment is a necessary step whenever an order is processed. This ensures that common steps are reused, promoting consistency and reducing redundancy.

On the other hand, the "extend" relationship allows a use case to add additional behavior under certain conditions, without modifying the base use case. This is useful for representing optional functionalities that extend the behavior of the main use case. For instance, a base use case "Search Product" could be extended by an "Offer Discount" use case that applies only when a certain condition, like a promotional event, is met. This allows the primary use case to remain simple, while still providing flexibility for conditional behavior.

Together, these relationships help in organizing use cases effectively, enabling better management of functionalities and requirements in a system.