Compulsory registration under GST: specified categories including inter State, casual, reverse charge and e commerce suppliers must register. Compulsory registration is required notwithstanding threshold rules for specified categories including inter-State suppliers, casual taxable persons, persons liable under reverse charge or sub-section (5) of section 9, non-resident suppliers, tax deductors under section 51, agents supplying on behalf of others, Input Service Distributors, electronic commerce operators, and suppliers of online information and database access from outside India to unregistered recipients.