Reversal of input tax credit: mandated debit and proportional calculation where supplies change or supplier fails to file returns. Reversal of input tax credit is required when a registered person moves to composition, supplies become wholly exempt, registration is cancelled, or other prescribed events occur; reversal is effected by debiting the electronic credit or cash ledger and must separately apportion central, state/UT and integrated tax. Rule 37A mandates reversal by recipients where suppliers fail to file corresponding returns within prescribed deadlines, with re availment allowed if the supplier later files. Rule 44 sets proportional calculation methods for inputs, semi finished and finished goods or capital goods, estimation where invoices are absent, certification requirements, and filing in prescribed forms; Rule 44A restricts re availment of certain customs duty credit on gold dore bar.