An Include connection indicates that the source element includes the functionality of the target element. Include connections are used in Use Case models to reflect that one Use Case includes the behavior of another. Use an Include relationship to avoid having the same subset of behavior in many Use Cases; this is similar to delegation used in Class models.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.641) states:
Include is a DirectedRelationship between two UseCases, indicating that the behavior of the included UseCase (the addition) is inserted into the behavior of the including UseCase (the includingCase). It is also a kind of NamedElement so that it can have a name in the context of its owning UseCase (the includingCase). The including UseCase may depend on the changes produced by executing the included UseCase. The included UseCase must be available for the behavior of the including UseCase to be completely described.
The Include relationship is intended to be used when there are common parts of the behavior of two or more UseCases. This common part is then extracted to a separate UseCase, to be included by all the base UseCases having this part in common. As the primary use of the Include relationship is for reuse of common parts, what is left in a base UseCase is usually not complete in itself but dependent on the included parts to be meaningful. This is reflected in the direction of the relationship, indicating that the base UseCase depends on the addition but not vice versa.
All of the behavior of the included UseCase is executed at a single location in the included UseCase before execution of the including UseCase is resumed.