Indiana Code Title 6. Taxation § 6-8.1-8-1.5

Sec. 1.5. (a) For a taxable period beginning after December 31, 2020, whenever a taxpayer makes a partial payment on the taxpayer's tax liability, the department shall apply the partial payment in the following order:

(b) For a taxable period beginning before January 1, 2021, whenever a taxpayer makes a partial payment on the taxpayer's tax liability, the department shall apply the partial payment in the following order:

In the case of a taxpayer with multiple liabilities, the department may adopt rules under IC 4-22-2 to establish the manner in which payments are applied to the taxpayer's outstanding liabilities.