Q. How are cutter numbers used by the Library of Congress?

Cutter numbers are an alpha-numeric code for the author's last name. It does not have to be the entire last name, but usually 3-4 letters of it.  This part of the call number further determines where the book is placed on the shelf.  There are also geographic cutter numbers, but author is what you will see most frequently.

Here's an example for the book "Thought Patterns for a Successful Career" by Lou Tice

HF5381 - Subject (Business Career Development)

.T53 - Cutter for last nameTice

2006 - copyright year

To further explore how Cutters are created, here is the Library of Congress Cutter Chart for your reference:

http://www.loc.gov/aba/pcc/053/table.html