When creating a new UML activity diagram, you are asked to specify a UML activity model for the diagram. All elements (action states, object states, and so on) belong to the model that you specify. In the activity model definition, you specify what classifier the model represents: a Use Case, a Class, or a Class Method.

Generally, you type the name of the activity diagram in the Activity Model property. Then click Define to define the Activity Model, and fill in the Use Case property, or the Class property, or the Operation property.

The model provides a separate namespace for Activity diagram elements. This enables you, for example, to create an Action State such as "Create Customer Record" when modeling a business Use Case, and create a separate Action State, different definition but same name, "Create Customer Record", when modeling an implementation method for an application.