For services its SAC(Services Accounting Code) and for Goods its HSN(Harmonized System Nomenclature).
Audit fee comes under Heading no. 9982 -Legal and accounting services
Group: 99822 Accounting, auditing and bookkeeping services
specific SAC for Financial auditing services is 998221
for Accounting and bookkeeping services is 998222.
You may mention no of digits of SAC in your invoice as per your turnover.
to find HSN or SAC codes with relevant tax rates also check with the updated tax rate notifications from