It is not related to the contents of any book, and should not be confused with Library of Congress Classification (LCC).
Although most of the bibliographic information is now electronically created, stored, and shared with other libraries, there is still a need to identify each unique record, and the LCCN continues to perform that function.
Librarians all over the world use this unique identifier in the process of cataloging most books which have been published in the United States.
There are also some peculiarities in numbers beginning with a "7" because of an experiment applied between 1969 and 1972 which added a check digit.
[5] The leading zeros padding the number are a more recent addition to the format, so many older works will show less-full codes.