What is a NAICS code, and how do I pick the right one when three seem to fit my business?

The primary NAICS code you select in SAM.gov does not restrict which federal contracts you can bid on, and you can change your primary NAICS code later if needed.

NAICS stands for North American Industry Classification System, which uses a six-digit hierarchical coding system that classifies all economic activity into twenty sectors. A solicitation will specify the NAICS code the contracting officer has determined is most appropriate for the work described in the solicitation. Your business must meet the size standard associated with that specific NAICS code for that particular solicitation to be considered a small business for that award. This means you could be a small business for one contract and an other-than-small business for another, depending on the assigned NAICS code and its corresponding size standard. The SBA size standards are set by NAICS code and are expressed either in number of employees or in annual receipts in millions of dollars. Your SAM.gov registration will remain active even if you change your primary NAICS code, which must be renewed every 365 days.