A Library of Congress catalog control number is a unique identification number that the Library of Congress assigns to the catalog record created for each book in its cataloged collections. Librarians use it to locate a specific Library of Congress catalog record in the national databases and to order catalog cards from the Library of Congress or from commercial suppliers. The Library of Congress assigns this number while the book is being cataloged. Under certain circumstances, however, a control number can be assigned before the book is published through the Preassigned Control Number Program.
Here are a few of the questions you can find the answer to at the PCN website:
How does the Preassigned Control Number (PCN) program relate to Copyright?
How much does it cost to obtain a Preassigned Control Number (PCN)?
Where do I send a copy of the book for which a Preassigned Control Number (PCN) was provided?
How can I get cataloging for a book which is already published?
What is the difference between the Cataloging in Publication (CIP) Program and the PCN program?
What is the difference between a Library of Congress Card Number, a Library of Congress Control Number, and a Preassigned Control Number?
Are Library of Congress catalog records created for all books submitted to the Library of Congress as part of the Preassigned Control Number Program?
How long does it take to obtain an account number and password?
How long does it take to obtain a Library of Congress Control Number?
What is an International Standard Book Number (ISBN)?
How do I get an ISBN?
What is an International Standard Serial Number (ISSN)?
How do I get an ISSN?