This registry entry sets the 2D option in the Construct Line 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 Line - 2D3D
Entry Type: Whole number - either 0 (Zero - False) to select the 2D option in the Construct Line dialog box, or the default value 1 (True) to select the 3D option.