The principal business code is a code that the IRS uses to "classify  sole proprietorships by the type of activity they are engaged in"

You would look for a code that most closely fits your self-employment income (1099-MISC income) and enter that.  If you can't find a code that fits, you can enter 999999.

Please click here and go to page 17 and 18.  Find the category closest to your income and then look at the sub-categories.

While inside the software and working on your return, type Schedule C in the Search at the top of the screen (you may see a magnifying glass there).  There will be a popup that says Jump to Schedule C.  Select that to get to the general area. 

·         Click to Edit next to the business

·         You should see Business Profile toward the top of the screen.

·         Select that, then scroll to Business Code