A 2d DXF export makes everything out of CAD 'lines'. You can of course [pre]adjust an arc or circle's segmentation, either as you create it [type Ns before entering the radius where N is the number of segments] or you can adjust it later on in 'Entity Info' - provided you haven't incorporated it into a 3d form, when it will become 'fixed'. You can also [pre]adjust the segment count in other SKP 'curve' types [again provided that they are no yet in a 3d form] by using 'Bezier Tools' [available from the 'Plugins Index' here...]. A 3d DXF export makes SKP arcs/circles as CAD real 'arcs'/'circles' [segmentation is ignored]; however other SKP 'curves' will remain as a collection of line segments.