Cenvat credit reversal: rules require provisional monthly reversals and a year end true up for mixed use of inputs and services. Rule 6 requires reversal or denial of Cenvat credit where inputs or input services are used for exempted goods or services, permits credit for exclusively dutiable use, and allows either separate accounts or specified payment options for mixed use. Monthly provisional reversals are payable on prescribed bases with a year end true up to compute actual reversal and adjust differences. Capital goods receive distinct treatment, and overriding provisions and SEZ or export payment conditions modify applicability and permit restoration of credit upon subsequent documented foreign exchange receipt.