What's the difference between NAICS and PSC codes?

They have a separate purpose and should be used appropriately.

NAICS classifies industries or similar business functions. They are largely used to determine small business set aside eligibility.

PSC (Product and Service Codes) are identifiers of the products and services provided. They serve as the first major filter for finding businesses for Government contracting opportunities.

They are used differently in market research in the Government contracting world. Using the right combination gives you incredible visibility to Government agencies looking to hire contractors.