The Library of Congress database classifies Literature (Belles-lettres) in the 800 section of the Dewey code. Many libraries do not use the numeric value but use "F" or "Fic" instead. In the Program Option window a replacement can be designated when the classification is in Literature (Belles-lettres).
Enter what you would like to use instead and it will be changed after the MARC information has been transferred to the MLM input form.
Also see Enable MARC