If you are selling goods to SEZ, there can be two cases: SEZ Exempt or SEZ Taxable
In both the cases, you need to select Interstate regardless of customer place of supply.
In case of SEZ Exempt, no tax account will be selected.
In case of SEZ Taxable, you need to select IGST tax account.
1. While making sales, select SEZ Exempt
2. It will be reflected in GSTR3B and GSTR1 section.
1. Make sure you have selected default tax account in item
2. Select SEZ Taxable while making sale
3. It will reflect in GSTR3B and GSTR1