Dear Bharat Ji,
In GSTR –1 the following insertion was made vide NN. 82/2020 - Central Tax dated 10.11.2020.
18. It will be mandatory to specify the number of digits of HSN code for goods or services that a class of registered persons shall be required to mention as may be specified in the notification issued from time to time under proviso to rule 46 of the said rules.
[Emphasis Supplied]
In the GSTR – 1, instruction no(s). 16, 17 and 18 seems for table 12 [HSN wise summary]
As you have rightly said that neither the rule (specifically) or the NN 78/2020 [only for invoices] states the requirement of HSN for Table No. 12.
However, the situation is a little dicey. The government vide form GSTR –1 has made this HSN Code mandatory to fill. So, the pertinent question being whether this mandatory requisite is in accordance with Rule 59 of the CGST Rules?
Relevant extract of Rule 59
(4) The details of outward supplies of goods or services or both furnished in FORM GSTR-1 shall include the–
(a) invoice wise details of all -
(i) inter-State and intra-State supplies made to the registered persons; and
(ii) inter-State supplies with invoice value more than two and a half lakh rupees made to the unregistered persons;
(b) consolidated details of all -
(i) intra-State supplies made to unregistered persons for each rate of tax; and
(ii) State wise inter-State supplies with invoice value upto two and a half lakh rupees made to unregistered persons for each rate of tax;
(c) debit and credit notes, if any, issued during the month for invoices issued previously.
[Emphasis Supplied]
If you look at GSTR –1, invoice wise detail (as stated in subrule 4 above) has to be filled in Table 4, 5 and 7 [This is coming from instruction no. 4 of GSTR – 1]
IF HSN was required to be filled in Table 4, 5 and 7, then read with instruction 18 of GSTR-1, in such situation, I would have said 100% it is mandatory and required to be filled.
However, for HSN they have made Table 12 separately. Now the interpretative question being whether Table 12 to be seen as a part of invoice wise detail as stated in Rule 59(4) of the CGST Act?
You may fight over interpretation and may establish that HSN is not required in light of rule 59(4). Or you may simply fill Table 12 and avoid any unnecessary dispute.