Determination of unpaid tax: notice, interest and tailored penalties with voluntary payment options to limit proceedings. Where tax is unpaid, short paid, erroneously refunded, or input tax credit wrongly availed for financial years from 2024-25, the proper officer must serve a show cause notice stating the tax, interest under section 50 and applicable penalty. Notices must be issued within 42 months from the annual return due date or erroneous refund date; related-period statements may be served if grounds match the earlier notice. The officer determines tax, interest and penalty after representation and issues an order within twelve months, subject to a sanctioned extension by a senior Commissioner-level officer. Voluntary payment options before or after notice alter penalty exposure and conclude proceedings where conditions are met.