Use Case Associations: Extensions and Uses
When the system to be described is large, it is useful to have modeling mechanisms that can be adapted to the desired level of detail. Associations between use cases provide this ability.
When a use case includes too many alternatives and exceptions, these are represented separately as relationships that extend the standard use case.
Inclusion relationship
One use case can be called automatically following another, for example validation of an order necessarily includes selection of a means of payment.
To indicate that one use case includes another:
1. In the use case diagram, click the Link button 
2. Click the use case, for example “Process Order" and drag the mouse to the case used, for example “Choose Payment Mode" before releasing the mouse button.
3. Select the link of type “Uses use case” and click OK.
The link appears in the diagram, labeled “Include”.
Examples of inclusion
In a training establishment, the following use cases:
• Inter-enterprise course (where the participants come from several different companies)
• Intra-enterprise course (where the participants all come from the same company)
can both include the following use case:
• Host and evaluate the course
Extend Relation
One use case can result in execution of another. Unlike inclusion, which is automatic, extension is optional.
To indicate that one use case is an extension of another:
1. In the use case diagram, click the arrow associated with the Link button and click Extension.
button and click Extension.2. Click the use case, for example "Consult Catalog" and drag the mouse to the extension case, for example "Process Order" before releasing the mouse button.
The Creation of Extension dialog box appears. You can define a constraint or an extension location.
3. Click OK.
The link appears in the diagram, labeled “Extend”.
Extension example
The purchase of an airline ticket can also include booking a hotel room or renting a car.
Extension point
The extension can intervene at a precise point in the extension case. This point is called the extension point.
To create an extension point on the extension case:
1. Open the properties window of the extension.
2. Select the Characteristics page.
3. In the Extension Point section, click Add.
The query dialog box appears.
4. Select the desired extension point and click Connect.
The extension point appears in the extension properties window.
An extension point can be associated with a constraint which indicates the moment at which the extension intervenes. You can add a constraint at creation of the extension or later, in the extension properties dialog box.
A constraint is a declaration that establishes a restriction or business rule generally involving several classes.Extension point example
The following example presents the use case of a bank transfer; above a sum of 20 euros, customer credit check is triggered.
