A Folium Surface looks something like a paraboloid glued to a plane. Divergence - the higher you set this value, the wider the paraboloid gets. Divergence - same as the 3d surface if you're revolving it around the Y axis. When zero, the two dimples punch right through and meet at the centre. Non-zero values give less dimpling.
Higher values make the two planes become closer.
The 2d curve that generates the above surface can be extruded in the Z direction or rotated about various axes by using the SOR parameters. In this case I've created the same shape as in the 3d version. When zero, the two dimples punch right through and meet at the centre. Non-zero values give less dimpling. Higher values make the two planes become closer.