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.
