Commodity Credit Corporation (CCC) loans.
In most cases, you don't report CCC loan proceeds as income. However, if you pledge part or all of your production to secure a CCC loan, you can elect to report the loan proceeds as income in the year you receive them. If you make this election (or made the election in a prior year), report loan proceeds you received in 2021 on line 5a. Attach a statement to your return showing the details of the loan(s).
See chapter 3 of Pub. 225.
Forfeited CCC loans. Include the full amount forfeited on line 5b, even if you reported the loan proceeds as income. This amount may be reported to you on Form 1099-A. If you didn't elect to report the loan proceeds as income, also include the forfeited amount on line 5c. If you did elect to report the loan proceeds as income, you generally won't have an entry on line 5c. But if the amount forfeited is different from your basis in the commodity, you may have an entry on line 5c.
See chapter 3 of Pub. 225 for details on the tax consequences of electing to report CCC loan proceeds as income or forfeiting CCC loans.
For additional instruction see http://www.irs.gov/pub/irs-pdf/i1040sf.pdf