Final answer:
The UML notation for the "includes" relationship is represented by a dashed arrow, highlighting the inclusion of one use case's functionality into another.
Step-by-step explanation:
The UML notation for the "includes" relationship is a dashed arrow. The "includes" relationship in UML is used to show that one use case (the base use case) includes the functionality of another use case (the included use case). The arrow is drawn from the base use case to the included use case, and the arrowhead is dashed to represent the inclusion. In UML diagrams, this illustrates modular behavior where certain actions are extracted into a separate use case for the purpose of reuse or clarity.