What is the purpose of 'extending' a use case?

The purpose of 'extending' a use case is to insert additional behavior into an existing use case without altering its fundamental structure. This allows for flexibility in how the use case functions, enabling developers to account for optional behaviors or added functionality that could enhance the user's interaction with the system. By using the extend relationship, you can define additional paths or scenarios that can occur alongside the base use case, usually in response to specific conditions or triggers.

This method is particularly useful in complex systems where not all behaviors are needed all the time but should still be defined for cases when they are. It helps maintain the integrity of the original use case while providing avenues for expansion, thus facilitating easier maintenance and understanding of the system as it evolves over time. In contrast, the other options either misrepresent the function of 'extending' or refer to processes that serve different purposes in use case management.