HSN is short form for Harmonized System of Nomenclature – a mechanism to uniquely classify and number supplies across the world in a systematic and logical manner. Internationally it is a 6 digit code. In India it is an 8 digit number (2 more digits are added).
SAC is Service Accounting Code – a unique code given to easily classify various services for identification and taxation. SAC is already present during Service Tax Regime and the same has been carried forward in GST regime.
HSN/SAC code must be mentioned on the invoice against the supplies/services offered by the society.