Ground Controls->Non_PDF->Trapping->Background
All Trapping settings under Background apply to the background color or fill of a text or image container. Text and Image containers with a background fill of none are not allowed by Quark to have trapping settings.
Overprint - This Ground Control will trigger if the background color or fill of a text or image container is set to overprint. This may cause problems if the container is over another object and the blending of colors may not be what the designer originally intended.
Knockout - This Ground Control will trigger if the background color or fill of a text or image container is set to knockout. If the container is on top of another item, the items color below the container will show through. This may pose a problem if the intent was to see the paper show through in the knockout area.
Auto - In Quark you can set a default value in the Preferences for Auto Amount. By default this value is 0.144 points. Whatever the value you set, the Ground control will trigger if a background color in a text or image container in Quark is set to either Auto Amount (+) or Auto Amount (-).
Custom - This Ground Control will trigger if the background color in a text or image container has been set to Custom.