Use the Solids tab of the Modify DWG/DXF Export Setup dialog to specify how solid geometry in 3D views is exported.
For Export solids as, select one of the following values:
A polymesh is a 3D shape consisting of multiple polygons that are meshed (joined) together. For example, a polymesh cube consists of 6 square surfaces that are joined together to form the cube shape. This geometry is referred to as face-based.
ACIS is a solid modeling technology. For example, ACIS represents a cube as a single object or shape with 6 sides.