What is meant by a use case that is classified as "extend"?

A use case classified as "extend" refers specifically to an optional behavior that is not always required for the primary function of the base use case but can be included under certain conditions. This means that when the primary scenario is happening, there is a possibility of triggering the extended behavior in response to a specific event or condition.

For instance, consider a use case for an online shopping system where the main flow involves a user adding items to a cart and proceeding to checkout. An "extend" use case could be a promotional offer that is presented if certain conditions are met, such as a total purchase value exceeding a specific amount. This promotional offer enhances or extends the functionality, but it only occurs optionally rather than being a mandatory part of the shopping process.

In this context, the other options do not accurately describe "extend" use cases. They don't encapsulate the essence of optionality that characterizes this classification. Option B, for instance, implies a mandatory part of the use case, which contrasts with the concept of extending functionality. Similarly, option C describes a part of the main flow rather than an option for supplemental behavior. Option D incorrectly suggests that all use cases can be classified as "extend," which is not true as they also include essential or