SKU is short for Stock Keeping Unit. The SKU is an identifier for products - knives in this case. Spyderco's SKUs can be divided into the following:
All SKUs used by Spyderco that begin with a "C" are reserved for folding knives (maybe derived from ClipIt)
SKUs starting with "FB" designate the knives of Spyderco's fixed blade line.
SKUs with a "K" at the beginning are reserved for Spyderco's kitchen knives