This dimension tool will show the length of the arc along the circumference. Don't confuse the arc length with the radius or diameter of the arc; there are other dimension tools for radius and diameter. To use this tool, click on the Arc Length option and then click on arc KL in the drawing and click at another point away from arc KL to place the dimension. You will now get an arc length dimension that will have an arc length symbol preceding it, as shown in the following screenshot:

The preceding arc length symbol will appear before the arc length dimension automatically but you can turn this symbol off in the Dimension Style Manager.