Chris,
the preferences have to do with how duties are imposed in NAFTA countries. Something made with all "American" materials will have ...
This site will explain it better than I can and with less typing
http://www.export.gov/logistics/exp_001034.asp
Essentially speaking, anyone importing parts from you would want preference criterion A or B, if your parts are made with "non-American" materials - they have to pay higher duties so you may not get the order. May not be very important to the average hobbyist but anyone exporting products (even skimmers and reactors) should have some familiarity with this.
James