I downloaded the NOV trial Diagram.  In the sample program I am in the Geometry Commands Section.  On this screen I see a circle Arc and a Elliptical Arc.  When I interact with two shapes there is a middle point with a tool tip that says "Modify base Point".  I can move this point on the line between the end points and then move the end points to flex the line in different ways.

If use the Arc Tool and Elliptical tool from the tool bar the tool tip on the mid points says "Modify base point" but I cannot slide the point across the line on the Arc Tool (I can on the Elliptical) and then move the endpoints to flex the line.  Please explain the difference between these controls.  

Also if there a way to select the first Arc I describe from the menu or does it have to be drawn programmatically?

Circle Arc Tool and Elliptical Arc Tool from the Tool bar

Geometry Commands  Circle Arc and Elliptical Arc