The Harmonised System of Nomenclature (HSN) is an internationally standardised system for classifying goods. Under GST, each product is mapped to an HSN code, which determines its tax rate. The number of HSN digits a business must show on invoices depends on its turnover, as notified.
Using the correct HSN code matters because it drives the GST rate applied; the equivalent classification for services is the SAC code.
The code is hierarchical, which is what makes it usable. The first two digits identify the chapter — the broadest family of goods — the next two the heading within it, and digits five and six the sub-heading. Up to that six-digit level the system is harmonised across member countries of the World Customs Organization; India extends it to eight digits for customs tariff items. Reading a code left to right therefore walks from a broad category down to a specific product.
Classification follows the product's composition and function, not its trade name and certainly not the rate the seller would prefer. The recognised general rules of interpretation settle hard cases — mixtures, sets, incomplete articles — and disputes over them are among the oldest arguments in indirect tax. The common practitioner failures are classifying by trade name rather than composition, carrying an obsolete code forward year after year, and rate shopping a friendlier heading, which invites demands for differential tax with interest.
Operationally the code lives in the item master: set once per product, it flows automatically onto invoices, into the e-invoice payload where the schema validates it against the published directory, and into the HSN-wise summary of GSTR-1, which must total back to the invoices reported. A mismatch between the summary and the invoice-level data is an easy notice trigger, so periodic review of the item master is genuinely cheaper than the correspondence it prevents.