To compress to a Grayscale color space.
The default setting for this parameter is false.
Note:
You have to make sure the image that you are exporting is actually a grayscale image. To make a Image Grayscale then use the Grayscale effect in the PictureEffects plugin.
If the image is not actually a grayscale image when it gets exported as grayscale, then the Red channel of the RGB color image is saved into the Grayscale Jpeg image (Making it only a gray representation of the Red channel).