But the plane definition is part of the arc definition…
When you mirror something the axis perpendicular to the mirror plane is inverted (i.e. if the mirror plane is the world YZ plane, the X axis is inverted, +x becomes -x). Because a right handed coordinate system is enforced, this 'flips" the plane’s Z axis to the other side, which indeed results in the arc direction (start>end) being reversed from that of the original object when viewed from the same direction.
Which is totally correct, but I have a feeling some 2D CAD programs do not have any concept of 3D and thus the ‘backwards’ facing Z axis is not taken into account, only the arc direction viewed from +Z.
I’m assuming that the ‘Flip arc normals to +Z’ setting was designed for exactly this situation - I just never really looked at it that closely, having never used a program that had a need for it.
@feklee I would just copy the DXF R12 export scheme and change the arc setting to ‘flip’ as above, then name the copy of the export scheme something like ‘Freecad’. Since there are so many programs out there that take .dxf files and each one has different requirements/quirks, you may find yourself using a number of different export schemes. What’s cool about Rhino is that you can configure the exports to do just about anything other programs need.