HSN code entry in GSTR 1

It was always mandatory to fill HSN summary. Even now it is mandatory for all suppliers

Yes , as per Notification 78/2020, the taxpayers having AG turnover less then 5 crore is required to mention 4 digit HSN & more the 5 crore its 6 digits.

Yes check has been put on HSN summary table no.12 of GSTR 1,
now you cannot file your GSTR1 without HSN or as per above mention.( it gives an error message while filing)

Is it mandatory for both B to B and C to C or only B to B

Both B2B & B2C

NO ITS NOT MANDATORY B2C IN CASE IF AG TURNOVER IS LESS THEN 5 CRORE.

Sir, is applicable B2B and B2C ?