Represents logical model relation from the extract source to the extract target.
In the concrete refactoring this reference can be represented as connector (composite structures),
object flow (activities), transition (states), message (interactions).
Sets type of the reference.
This type is used to type the elements which represent the reference in the model.
For example, in activity extract the type is used to type activity parameters, activity parameter nodes,
pins. In extract structure it is used to type ports.