The first step of this is google-able.  Per irs.gov form instructions:

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.

So you'll have to see if this applies to your client.  The items further down on the K-1 don't always fit neatly into K-1 input boxes on the personal returns, so it takes extra due diligence form you to 1) ask you client for relevant facts and circumstances (that they will likely be clueless about), and 2) deep dive into further research.

Good luck!