Hsn - invoice

Under GST, the majority of dealers will need to adopt two-, four-, or eight-digit HSN codes for their commodities, depending on their turn over the previous year.

If you are required to adopt HSN code then adoption will be seen in this way that the dealer at the time of registration/migration under GST, will be required to mention Harmonized System of Nomenclature (HSN) code of the goods he deals with & Services Accounting Code (SAC) Code for Service for which he is liable to pay tax.

However, read this which is taken from FAQs on GST hosted by CBEC

"Q 20. How will the goods and services be classified under GST regime?

Ans. HSN (Harmonised System of Nomenclature) code shall be used for classifying the goods under the GST regime. Taxpayers whose turnover is above Rs. 1.5 crores but below Rs. 5 crores shall use 2-digit code and the taxpayers whose turnover is Rs. 5 crores and above shall use 4-digit code. Taxpayers whose turnover is below Rs. 1.5 crores are not required to mention HSN Code in their invoices"

Thus what CBEC is saying that if you are required to take HSN Codes then you are also required to post the code on Invoice as well.