If you have not set the
Use raster image colour
depth switch in the
Raster graphics part of the dialog, you
have to select a colour depth for all raster graphics here. This is
recommended if, say, you want to export a PS file for a b&w printer.
Even if your document contains many colour images, you should choose
Another use: If you want to e.g. convert your PS files to PDF for screen display only, it might be better not to use CMYK colour depth. In that case it is better to select
CMYK separated is not selectable, then
this may be due to the fact that you have only activated the
PostScript Level 1 switch in this function group. CMYK is
supported in PostScript Level 2 for the first time!