A use case, a concept invented by Ivar Jacobson at 1992, is a sequence of transactions performed by a system that yields an outwardly visible, measurable result of value for a particular actor. A use case typically represents a major piece of (end-to-end) functionality that is complete from beginning to end.

Use case diagrams are used during requirements elicitation and analysis as a graphical means of representing the functional requirements of the system. Use cases are developed during requirements elicitation and are further refined and corrected as they are reviewed (by stakeholders) during analysis. Use cases are also very helpful for writing acceptance test cases. The test planner can extract scenarios from the use cases for test cases.

Note: A use case describes how a type of user (or known as an actor) uses a system to achieve a particular user goal.

ATm - actor, system and goal

Thus, there are three key things we need to know to describe a use case:

  • The actor or actors involved. An actor is a type of user (for example, cardholder) that interacts with the system.
  • The system being used.
  • The functional goal that the actor achieves using the system the reason for using the system.

Use Case at a Glance

A use case diagram is a visual representation of the relationships between actors and use cases together that documents the system’s intended behavior. There is no rocket science to it at all: a usage case is simply a reason to use a system. For example, a bank cardholder might need to use an ATM to get cash out of their account. It is as simple as that.

ATM use case diagram

Elements of Use Case Diagram


Notation Description


Visual Representation


  • Someone interacts with a use case (system function).
  • Named by a noun.
  • Actor plays a role in the business
  • Similar to the concept of user, but a user can play different roles
  • For example:
    • A prof. can be an instructor and also a researcher
    • plays 2 roles with two systems
  • Actor triggers use case(s).
  • Actor has a responsibility toward the system (inputs), and Actor has expectations from the system (outputs).
Use Case Diagram symbol: actor
Use Case

  • System function (process – automated or manual)
  • Named by verb + Noun (or Noun Phrase).
  • i.e. Do something
  • Each Actor must be linked to a use case, while some use cases may not be linked to actors.
Use Case Diagram symbol: use case
Communication Link

  • The participation of an actor in a use case is shown by connecting an actor to a use case by a solid link.
  • Actors may be connected to use cases by associations, indicating that the actor and the use case communicate with one another using messages.
Use Case Diagram symbol: communication link
Boundary of system

  • The system boundary is potentially the entire system as defined in the requirements document.
  • For large and complex systems, each module may be the system boundary.
  • For example, for an ERP system for an organization, each of the modules such as personnel, payroll, accounting, etc.
  • can form a system boundary for use cases specific to each of these business functions.
  • The entire system can span all of these modules depicting the overall system boundary
Use Case Diagram symbol: system

Structuring Use Case Diagram with Relationships

Use cases share different kinds of relationships. Defining the relationship between two use cases is the decision of the software analysts of the use case diagram. A relationship between two use cases is basically modeling the dependency between the two use cases. The reuse of an existing use case by using different types of relationships reduces the overall effort required in developing a system. Use case relationships are listed as the following:


Use Case Relationship


Visual Representation


  • Indicates that an “Invalid Password” use case may include (subject to specified in the extension) the behavior specified by base use case “Login Account”.
  • Depict with a directed arrow having a dotted line. The tip of arrowhead points to the base use case and the child use case is connected at the base of the arrow.
  • The stereotype “<<extends>>” identifies as an extend relationship
Use Case Diagram symbol: extend

  • When a use case is depicted as using the functionality of another use case, the relationship between the use cases is named as include or uses relationship.
  • A use case includes the functionality described in another use case as a part of its business process flow.
  • A uses relationship from base use case to child use case indicates that an instance of the base use case will include the behavior as specified in the child use case.
  • An include relationship is depicted with a directed arrow having a dotted line. The tip of arrowhead points to the child use case and the parent use case connected at the base of the arrow.
  • The stereotype “<<include>>” identifies the relationship as an include relationship.
Use Case Diagram symbol: include

  • A generalization relationship is a parent-child relationship between use cases.
  • The child use case is an enhancement of the parent use case.
  • Generalization is shown as a directed arrow with a triangle arrowhead.
  • The child use case is connected at the base of the arrow. The tip of the arrow is connected to the parent use case.
Use Case Diagram symbol: generalization

<<include>> vs <<extend>> use case

It is common to be confused as to whether to use the include relationship or the extend relationship. Consider the following distinctions between the two:

  • Use Case X includes Use Case Y: X has a multi-step subtask Y. In the course of doing X or a subtask of X, Y will always be completed.
  • Use Case X extends Use Case Y: Y performs a sub-task and X is a similar but more specialized way of accomplishing that subtask (e.g. closing the door is a sub-task of Y; X provides a means for closing a blocked door with a few extra steps). X only happens in an exceptional situation. Y can complete without X ever happening. In general, the extend relationship makes the use cases difficult to understand. It is suggested that developers use this relationship sparingly.

Use Case Levels of Details

Use case granularity refers to the way in which information is organized within use case specifications, and to some extent, the level of detail at which they are written. Achieving the right level of use case granularity eases communication between stakeholders and developers and improves project planning.

Alastair Cockburn in Writing Effective Use Cases gives us an easy way to visualize different levels of goal level by thinking in terms of the sea:

Use case levels

Note that:

  • While a use case itself might drill into a lot of detail about every possibility, a use-case diagram is often used for a higher-level view of the system as blueprints.
  • It is beneficial to write use cases at a coarser level of granularity with less detail when it’s not required.

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Use case diagram example

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