The principal business code is a code that the IRS uses to "classify sole proprietorships by the type of activity they are engaged in ..."
So, 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 go to the link below and go to page 17 and 18. Find the category closest to your income and then look at the sub-categories.
https://www.irs.gov/pub/irs-pdf/i1040sc.pdf
