Codes can be useful in retrieving information from the US Government or other databases. Knowing an industry code or a tariff code allows you to do a targeted search in any of the library databases which will yield better results.
NAICS - The North American Industry Classification System (NAICS) is the standard used by Federal statistical agencies in classifying business establishments for the purpose of collecting, analyzing, and publishing statistical data related to the U.S. business economy.
The more digits you use, the more focused your search becomes. Fewer digits create a broader, more general search.
NOTE: For smaller markets, you may not find the exact NAICS code. Therefore you might need to explore two different codes and explore the ways your project might be viewed in more than one market segment. Sometimes your niche may not really fit into NAICS.
Search several example or test companies in AtoZ above and you will see many NAICS and SIC codes.
Test the codes to see which one fit your project best.