For information on which Business Activity code to use on Schedule C, refer to the IRS website.
The codes for the Principal Business or Professional Activity classify sole proprietorship by the type of activity they are engaged in to facilitate the administration of the Internal Revenue Code. These six-digit codes are based on the North American Industry Classification System (NAICS).
Select the category that best describes your primary business activity (for example, Real Estate). Then select the activity that best identifies the principal source of your sales or receipts (for example, real estate agent). Now find the six-digit code assigned to this activity (for example, 531210, the code for offices of real estate agents and brokers) and enter it on Schedule C or C-EZ, line B.
Note: If your principal source of income is from farming activities, you should file Schedule F.