You may have overlapping colored objects in your PDF document, for example
text or an image on a colored background. If so, you can specify
what should happen with these colors when they are printed:
- Knockout, meaning that the colors of the object in
the foreground cut out the area underneath. In other words, the background
color is erased and the resulting color will be the foreground color.
- Overprint, meaning that the colors of the object are printed
on top of the background colors. The resulting color is a combination
of the foreground and the background color.