A quick search shows that some CAD programs use "Spline Leaders," and "Arc Leaders." The arc leader in Autocad had a slightly curved arrow.

First time to play with the Dim's. They are elegant and well thought out, likely representing lots of work. There is much to learn. Changing arrows, and options, and presets, is not understood...

- Brian

So a guess would be that dimensions represent a special kind of geometric object, not subject to many of MoI's commands?