According to the official documentation the quality parameter can be 1 - 100 and the effect varies depending on the resulting format.
For JPG output a value of 1 gives worst quality and highest compression; a value of 100 gives best quality and lowest compression. For PNG files the quality parameter only changes the lossless compression strategies used, so the speed of compression and the resulting file size can be affected but image quality remains the same.
With any Linux command, you can enter
man [command]
To see the help page that list out the information and parameters available."
