The IRN (hash) will be generated by GST System using GSTIN of supplier or document creator, financial year and the unique serial number of the document/invoice. The IRN can also be generated by the seller. The serial number of invoice will be unique for a GSTIN for a Fin Year and the same has to be captured by Supplier’s ERP.Supplier has to keep the IRN against each of its invoice. It will be advisable to keep the same in the ERP as invoice without IRN will not be a legal document.