Probably those records were imported to Library of Congress from other sources, because they are assignments that public libraries sometimes make. Dewey put fiction in the 800s, but because a public library carries so much fiction and it is so popular, we tend to use Dewey for nonfiction (and perhaps classic literature, poetry, essays and humor), and we pull fiction out by itself alphabetizing by author. So FIC means that rather than shelving by Dewey, the book has been shelved by author name in a section labeled FIC.
Dewey also originally put biographies in the 920s, which grouped them all together, and which suited public libraries. Recently, though, OCLC (who own DDC), stopped using the 920s (except 920 - group bios and 929 - genealogy), and now biographies are shelved with their discipline (so Einstein's biography is in the 500s with Science, and Joe Namath would be in the 790s and sports). The letter B denotes that it is a biography. Some public libraries use only the letter B (or the defunct 920s) to still keep biographies together.
Other non-Dewey labels sometimes used in public libraries, and possibly stored in the Dewey field:
E - Easy reader (used for picture books)
B or BR - Beginning reader (B may also be a biography, however)
J - Juvenile fiction
YA - young adult fiction
F or FIC - adult fiction
Sometimes formats may be put in this field - DVD, AUD, CD, LP (large print).