This dimension tool will add the radius dimension on a circle or arc. In the following example, we will add the radius dimension on arc KL of the sample drawing: 

1. Select the Radius tool from the dimension drop-down menu and then click on arc KL, then move your cursor away from arc KL and click again to place the dimension.
2. The radius dimension will be added now, and the dimension will have the R prefix, indicating the radius value.
3. You can add a radius dimension to a circle as well, using a similar workflow: 

After adding the radius dimension on the arc, my dimension looked like the preceding screenshot.