A use case diagram example consists of four use cases: place order, check status, fill orders and establish credit. Order can be placed directly by customer or a salesperson and they can also check the order status later on. Shipping clerk is responsible for filling the order and the supervise can establish the credit level for customer according the past record and financial information submitted by the customer. Now let us refine the main use case – place order
Include Use Case - The include and extend relationships are drawn as dashed arrows with the keyword «include» or «extend». The include relationship points at the use case to be included; the extend relationship points at the use case to be extended.
Extend Use Case - A use case can also be deﬁned as an incremental extension to a base use case. This is called an extend relationship. There may be several extensions of the same base use case that may all be applied together.
Child Use Case - Use case generalization is drawn the same as any generalization, as a line from the child use case to the parent use case with a large triangular arrowhead on the parent end.