The Library of Congress number is used by the United States' central library to track submissions for the library's permanent collection. We'll submit a copy of your book to the Library of Congress, and arrange for a card catalog number to be assigned to your book.
The Library of Congress does not provide catalog numbers for books under 50 pages unless they are a children's book. They also reserve the right to refuse to catalog.
Note: If this service is purchased after your book has been published, additional fees may apply.