Once a PCN is issued, the publisher of the book prints the control number inside the book. That preassigned Library of Congress reference assists libraries and book sellers in cataloging the book. This is an optional service that identifies your book in the master catalog of the Library of Congress. When your book is published, one copy is to be sent to the Library for the Congressional Archives.
Books that have previously been published and paperback books are not eligible for the program, which is available only for books published in the United States.
To have a PCN assigned to your book, there is an application that must be filled out and filed with the Library of Congress. There is no fee charged during the application process.
The PCN program has nothing to do with copyrighting your work. A copyright is used to document the creative ownership of a work, while the PCN is simply the assignment, in advance of a cataloging number by the Library of Congress.
One of the nice aspects of the PCN program is that it is available even to single-book publishers, while other Library of Congress control numbers require multiple titles to be in circulation.