Compulsory registration required for inter state suppliers, casual and non resident taxable persons, reverse charge and e commerce operators. Compulsory registration applies to specified classes irrespective of threshold, including inter state suppliers, casual taxable persons, persons liable to pay tax under reverse charge, electronic commerce operators required to pay tax, non resident taxable persons, tax deductors, agents supplying on behalf of registered persons, input service distributors, persons required to collect tax, and persons supplying online information and database retrieval services from outside India to unregistered persons; additional classes may be notified by the government.