In term of Notification 78/2020 CT , the applicability of HSN is NOT accordance to classification of Goods . It applicability is as per Aggregate Turnover of Registered Person.
Hence even if it is Exempted or Nil rated HSN is mandatory.
Kindly Refer the Query again " HSN code for Exempted Goods" & petrol /diesel are not cover under GST (Section 9(2) CGST Act) , it's under VAT regime.
So vat invoice is issued for petrol & diesel . Hence HSN is not required therein. But Querist has query in regard to exempted goods not on Non GST Goods , so the reply was accordingly given.
For exempted goods HSN code is not required because GST rate is compulsorily required in the said table & for exempted goods no question of GST rate as it is exempted & we Cannot put Zero which is for Export only