Subject of a Use Case represents a system under consideration to which that Use Case applies. The subject can be any element that may have a behavior, that is, a Component or Class (SysML Block).
To specify the Use Case subject
Select the Use Case or its shape.
Drag it onto a Class, SysML Block, or Component shape. The Use Case becomes nested in that shape, and the appropriate model element is set as the Use Case subject.
Having a Use Case with the subject specified jump-starts the creation of Swimlanes in the Activity diagram owned by that Use Case. In this case, you can select the subject properties you wish to be represented by the Swimlanes on that diagram.