The Harvey Library organizes its collections by subject. These subjects are represented by a call number system called the Library of Congress Classification System (LC). LC uses call numbers that begin with letters.

If you were looking for the book 'The Legend of Cessna' by Jeffrey L. Rodengen using the online catalog you would find that the call number is TL686. C4 R63 2007. But what does this number mean?

TL	The first section can be one to three letters. TL items are about technology, specifically motor vehicles, aeronautics, and astronautics.
686. C4	The second section describes the subject in more detail (in this case - aiplanes)
R63	The third section describes the author. The R stands for the author's last name (Rodengen)
2007	The book was published in 2007