If you are referring to principal business activity codes, they are contained within a learn more link in the program.
But, you can also easily find them at the link below starting on page C-17 - 18. It is a six digit code that is used for statistical purposes - pick the category that most closely resembles your business activity. If you can't find anything close enough to your business activity, then you should use the misc code "999999"
https://www.irs.gov/pub/irs-pdf/i1040sc.pdf
