This registry entry sets the 2D option in the Construct Circle dialog box if it is set to 0 (Zero - False). It sets the 3D option if it is set to 1 (True).
Entry Name: Constructed Circle - 2D3D
Entry Type: Whole number - either the default value 0 (Zero - False) to select the 2D option in the Construct Circle dialog box, or 1 (True) to select the 3D option.