If the S corp has only one member / owner, line 16 would equal IRS Form 1120-S Schedule L line 19 Loans from shareholders.
IRS Form 7203 Instructions page 3 states:
Line 16
Enter the balance of each loan to the S corporation at the beginning of the corporation’s tax year in a separate column.
Line 21
Enter the debt basis of your loan(s) to the S corporation at the beginning of the corporation’s tax year.
What is a Debt Basis?
Measuring a shareholder's debt basis is similar to measuring a stock basis. To calculate a debt basis, you take the original amount the stockholder loaned to the corporation and increase his or her basis for that loan and any additional loans he or she provided.
Passthrough items of income or gain generally increase debt basis. However, the items of income, gain, loss, and deduction must first be netted, along with the non-dividend distributions. Reductions in debt basis result in gain when the loan is partially or fully repaid.