I am going GUESS that you are referring to the 2020 

See page 22 of the shareholder instructions for the k-1 https://www.irs.gov/pub/irs-dft/i1120ssk--dft.pdf

Also list of codes on page 24

Code AC. Gross receipts for section 448(c) Use the gross receipts
amount to figure the business interest
expense you can deduct, if
applicable. See section 163(j) and the
Instructions for Form 8990 for details.