GSTN has clarified as follows-
Specific 6-digit HSNs, as available in the HSN/Customs Tariff (with the corresponding description of goods) are allowed in the system. It also follows that the declaration of HSN at 4/6 Digits has to be out of valid HSN codes only.
Where HSN 6 digits are specified to be reported in the invoice, valid HSN codes as available in tariff, at both 6-digits and 8-digits can be mentioned. Similarly, where HSN at 4-digits a
re specified, valid HSN codes as available in tariff, at 4-digit, 6-digit, and 8-digit can be mentioned. However, the 4/6 Digit HSN Codes, which are not available in the tariff; along with specific description, Unit, and GST Rate; are not allowed to be mentioned.