Understanding include and extend relationship using use case example.

Include: When one or more use cases include the flow of another use case, this is referred to as an include relationship. For example, if I want to buy a product then I have to pay for it.
Note: The flow of that use case is always included, and the arrow should be towards the use case being included, it is represented by the dotted line.

Extend: When one or more use cases include the flow of another use case conditionally, means the flow of that use case might always not be executed.
For example, we want to buy a product from an e-commerce company then we have to first login but if we are already logged in then there is no need to include the flow of login use case.