This value indicates the maximum number of darkness levels
supported by this printer.
The level settings range from 1 to xcmPrtGeneralDarknessLevels
where 1 represents the lightest level and
xcmPrtGeneralDarknessLevels represents the darkest level.
The value 1 implies no user settables.
The value 0 means unspecified.
The selection of a darkness levels specifies that the
printer shall process the page images so that they appear
either 'lighter' or 'darker'.