The «include» and «extend» Relationships in Use Case Models https://www.youtube.com/watch?v=zid-MVo7M-E
Use Case model is used to specify the functionality of a system from the point of view of the business users. Each Use Cases describes a logical task that may be performed by the system. The Use Case description describes the interaction between the system and the outside world. That interaction may be an online transaction where the Actor is a human user.
We use UML as an effective tool to create a specification for a system. A development process that is aimed at new build can be adapted quite straightforwardly for enhancing existing systems or evaluating and implementing packages.
A Use Case describes the behaviour of a business system from the business user’s point of view2. It should describe in plain business terms how the user interacts with the system (assuming it is an online Use Case) and what the system does in response. It does not determine how the system works internally, that is it does not define the implementation. The description of a Use Case is usually presented in text.