Differential tax liability: remit shortfall with interest or furnish acceptable explanation within seven days under rule 88C. Intimation notifies that tax declared in FORM GSTR-1/GSTR-1A/IFF exceeds tax reported or paid in FORM GSTR-3B, creating a differential tax liability. Taxpayer must, within seven days, either pay the differential with interest under section 50 via FORM GST DRC-03 and furnish ARN in Part-B, or submit reasons for unpaid portions (including excess earlier-period payments, late declarations, amendable filing errors, or advance adjustments). Failure to pay or provide an acceptable explanation permits recovery under section 79. The reply requires verification and authorized signature.