You can add a comment to the activity diagram to make your diagram easier to read and to provide additional information. This has no effect on the execution, it is just for documenting purposes.
Activities are modeled in activity diagrams. Refer to Modeling Activities for more information on how to create an activity diagram.
Drag out the comment icon from the elements toolbar. You can place the comment anywhere within the diagram pane at a desired position.
Once you have placed the comment, you can enter a text to it directly in the diagram by double-clicking the element.
By double-clicking in the element, you can also change or adjust the text of the comment directly in the diagram. Alternatively, you can switch to the Attributes panel to change the text of the comment.