Compulsory registration required for inter-state, reverse-charge, casual and electronic suppliers under GST registration rules. Compulsory registration is mandated for specified categories regardless of turnover: inter-State suppliers, casual and non-resident taxable persons, persons liable under the reverse charge mechanism or required to pay tax under the deemed-supply provision, persons required to deduct tax, agents supplying on behalf of others, Input Service Distributors, electronic commerce operators, suppliers of online information and database access from outside India to unregistered Indian recipients, suppliers through e-commerce operators required to collect tax at source, and any other classes notified by the Government on Council recommendations.