Use case diagrams are usually created at the requirements documentation phase. They are used to depict the functions of a system and the actors involved in using those functions. They are depicted using a specific notation, which ensures that there is no ambiguity in what you’re trying to convey. They have an advantage over a text-based description which can be open to interpretation, which could then cause problems in the delivery of the chosen solution.
What is a use case diagram?
A use case diagram can be used to show the top-level view of a business system and are used to show the things that need to be provided at an organisational, business system or IT system level. Use case diagrams are used to depict the functions of a system and the actors involved in using those functions. A function may be defined as a set of actions that the business users want the IT system to support in order to achieve a specific goal.
For example, the use case diagram may provide a depiction of a customer account system with the various functions that the business users want the system to support in order to achieve a specific goal. A function may be described using the verb/noun rule, such as ‘record customer’.
Why use a use case diagram?
Use case diagrams use a specific notation, which ensures that there is no ambiguity in what you’re trying to convey. If you use a text-based description there is potential for somebody reading it to interpret your message differently which could cause problems in the delivery of the chosen solution.
How is a use case diagram created?
Unified Modelling Language (UML) is the notation most commonly used in use case diagrams. Use case diagrams each describe an overarching objective and contain descriptions of the various actions that the system must perform to achieve this objective. They document the interaction between an actor and the system. Placing a boundary around the system with the actors placed outside allows you to clearly demonstrate the system boundary and that the scope that it must work within.
Use cases are usually created at the requirements documentation phase. You should aim to work alongside the relevant actors within the business in order to fully comprehend the relationships between the various people and systems. You can elicit information about the relationships through workshops, where you ask the attendees to discuss their roles and functions within the business system.

- The diagram shows use case diagram for a library booking system.
- There is a rectangle with three circles within which are borrow book; administrate accounts; open account.
- On the outskirts of the rectangle there are three people librarian; customer and system administrator.
- There are then arrows from each of the circles to show the links between the people and the process. From the borrow book circle then there is an arrow to the librarian saying ‘facilitate borrow’ and then one to customer saying ‘request book’. From administrate accounts there is an arrow to the system administrator. There is an arrow from open account to the customer.
Things to consider
Further details can be captured in a use case description which is a textual description of the interactions between an actor and a use case.