Compulsory registration: inter state suppliers, reverse charge payers, non residents and e commerce operators must register. Compulsory registration applies to specified categories: persons making inter state taxable supplies, casual taxable persons, reverse charge payers, persons required to pay tax under the specified supply provision, non resident taxable persons, persons required to deduct tax, agents or persons supplying on behalf of others, Input Service Distributors, suppliers using electronic commerce operators required to collect tax at source, every electronic commerce operator, suppliers of online information and database services from outside to unregistered persons in India, and other notified classes.