Compulsory registration required for inter state suppliers, casual and non resident providers, reverse charge and e commerce operators. Compulsory registration is required for specified categories irrespective of the general registration threshold: inter State suppliers, casual taxable persons, persons liable under reverse charge, persons liable under special charging provisions, non resident suppliers, tax deductors, agents or persons supplying on behalf of others, Input Service Distributors, electronic commerce operators and persons supplying online data services from outside India to unregistered persons; the Government may notify other classes and registration is required whether or not some categories are separately registered under the Act.