IRS business activity codes are for your principal business or professional activity. For a list of business activity codes, see Instructions for Schedule C.
Choose the category that best describes your sole proprietorship’s primary business activity (for example, real estate) and select the activity that best identifies the source of your sales or receipts (for example, real estate agent).
Once you've determined the best code for your business (for example, 531210, the code for offices of real estate agents and brokers), enter it on Schedule C, Line B.