Typically, it is explained to you in additional information (supplementary) that should have been provided to you along with your K1. If you did not receive this additional supplemental information, and do not wish to or are not able to contact the provider of the K1, here are some bit of information that will help guide you through a better understanding of this code as it relates to Box 17 on your K1...
IRS K1 specifically pages 20-24
BOX 17: 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.
The section 163(j) limitation is applied at the partnership level. As provided in Q/A 1, the amount of deductible business interest expense in a taxable year cannot exceed the sum of the partnership's business interest income, 30% of the partnership's ATI, and the partnership's floor plan financing interest expense. Sep 24, 2020
IRS Section 163(j)
IRS Instructions for Form 8990
