Declaration of GSTIN mandatory in customs documents to enable IGST credit and GST refund eligibility. Declaration of a valid GSTIN in customs documents is mandatory from GST commencement to enable IGST credit on imports and GST refund on exports; the GSTIN will be validated for correct IEC/PAN linkage, and where PAN discrepancies exist between GST registration and IEC, amendment of PAN in IEC must be undertaken, with a special GSTN updation window provided to facilitate corrections.