Hi Debbie.
The letters refer to a number that is obtained by summing each of the numbers in the table. There is an explanation of it in video here:
http://the-checkout-tech.com/tubetorials.php
Click on the XE-A102 vids as this machine is similar as far as programming.
Also, if anyone else needs a Sharp XE-A106 manual, go here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp9.php
Registerman
June 2010