IGST apportionment where place of supply is unknown requires distribution among States and Union Territories by prior supply ratios. Apportionment of IGST collected from taxable persons where the place of supply cannot be identified, requiring reporting of the supplier's GSTIN, total IGST collected, CGST portion, and the balance to be distributed among States and Union Territories. Allocation is to be made in the same proportion in which supplies were made to each State or Union Territory in the previous period, for distribution of the SGST/UTGST share of IGST under section 17 of the IGST Act.