Prefer Exact Trim
This menu item determines which trim type (Exact or Proportional to Paper Size) will be used if the Dynamically Trim and the Proportional Aspect Ratio menu items are checked. Then if the Prefer Exact Trim menu item is checked, the Exact trim type will be used for dynamic trimming. Otherwise the Proportional to Paper Size trim type will be used.
Also note that if the Proportional Aspect Ratio item is not checked, the trim type is always Exact.
For more information about trimming see the Trimming topic.