Use Case Relationship

Key: forget about OOD concept

 You read <> associations in the direction of the arrow,while you read <> associations in the reverse direction. 

Generalization (denoted Use case B is a type of use case A)

it as an “overrides” relationship. it is just saying that these use cases are all "general" type of use cases. general case is just a concept instead of real implementation. e.g. pay, can have pay by credit, pay be paypal as special cases, you only need to implement pay by credit ad pay by paypal.

A<>B

similar to function call. half way through A, B is called. after B is finished, A continues its rest part.

A<>B

this is not inheritance.B is basic and complete use case, A is additional function can be added to B.

A<B

A happens before B

A<B
similar to includes