HSN code requirement on tax invoices: digit length tied to annual turnover tiers, affecting registered persons' invoicing obligations. Requires registered persons to include specified digits of the Harmonised System of Nomenclature (HSN) Code on tax invoices according to their annual turnover in the preceding financial year: nil for the lowest turnover tier, two digits for the intermediate tier, and four digits for the highest tier. The requirement follows the proviso to the invoicing rule under the GST Act and takes effect from the prescribed commencement date, creating a turnover-linked HSN disclosure obligation for tax invoices.