A curve through three points, for a leader that has to bend around something.
The Arc draws a curve from three clicks. Use it where a straight leader would cross something it
should not: another callout, a dimension string, the detail itself.
Draw it: click the start, click a point the curve must run through, click the end. It
finishes itself on the third click; there is no Enter to press.
Keys while placing
Backspace: remove the last point placed.
Escape: finishes with what is placed rather than discarding it.
Start ending / End ending: the same nine styles, both bare by default.
Notes
The middle click is a point ON the curve, not a control handle off to one side. Put it where the curve should actually pass.
A head follows the curve's direction where it lands, not the straight line between the two ends, so a curved leader reads as arriving at its target.
An arc has exactly three points and keeps them: selecting it gives a handle on each, with no way to insert or remove one.
It is stored as those three points in click order. A viewer that does not know this tool reads them as a two-segment run through the same points, which approximates the curve rather than misdrawing it.