A use case is made up of a variety of sections; not all use cases will have requirements to fulfill each section. These sections are:

1.    Use Case Name

2.    Actors

3.    Summary

4.    Pre-Conditions

5.    Flow Of Events

6.    Error Conditions

7.    Post-Conditions

As an example, I will take the example ATM System Usage Use Case in Figure 1 and break it down into the above sections.

Use Case Name

ATM System Usage

Actors

Custom, Bank ATM

Summary

A customer withdraws cash from the ATM by inserting his bank card into the ATM, entering a correct PIN number to access the system, and selecting and entering the amount to be withdrawn. The ATM then validates the card and PIN, and the funds are available for withdrawal. If the information provided by the customer is valid, then the ATM dispenses the money and returns the customer's card.

Pre-Conditions

1.    The ATM is in working order.

2.    The ATM has cash.

Flow of Events

1.    The customer inserts the ATM card.

2.    The ATM reads and validates the card.

3.    The customer enters the PIN number.

4.    The ATM validates the PIN number.

5.    The user selects and enters the amount to withdraw.

6.    The ATM validates that the funds are available.

7.    The ATM dispenses the cash.

8.    The ATM returns the card to the customer.

9.    The customer removes the card and the cash.

Error Conditions

1.    An incorrect card is entered.

2.    The PIN is incorrect.

3.    The account has insufficient funds.

Post Conditions

1.    The customer's account balance is adjusted

2.    The ATM's cash supply is adjusted

As you can see, there are quite a few actions that happen when a customer wishes to make a cash withdrawal. Keep in mind that there are even more steps you could add, but for the purpose of this article, this should provide the basic idea as to what makes up a use case.