Edit Bar controls: Fixed Text, Width, Height, Settings, Favourites.
The tool allows you to dimension the length of an arc or circle.
See Dimension General for standard Edit Bar controls.
Pick the arc to be dimensioned and a flexible dimension, comprising a projection line, dimension line and text outline will be drawn, allowing you to pick a point to position the dimension. If the Fixed Text option is switched off you should then pick a point to position the text.