Compulsory registration required for specified suppliers including inter-state, casual, reverse-charge payers and electronic commerce operators. Compulsory registration is mandated for specified categories irrespective of threshold: persons making inter-State taxable supplies, casual taxable persons, persons liable under the reverse charge mechanism, persons required to pay tax under subsection (5) of section 9, non-resident taxable persons, persons required to deduct tax under section 51, agents supplying on behalf of others, Input Service Distributors, suppliers using electronic commerce operators required to collect tax at source, every electronic commerce operator, persons supplying online information and database services from outside India to unregistered recipients, and any other persons notified by the Government.