Registration threshold: mandatory GST registration applies when aggregate turnover exceeds the prescribed limit, including special state concession. Suppliers of taxable goods or services must register when their aggregate turnover in a financial year exceeds the Act's threshold, with a lower threshold for supplies from special category States; persons already registered or licenced under prior law must register from the appointed day. Where a registered taxable person's business is transferred or succeeds to another, the transferee or successor must register from the date of transfer, and where transfer follows an approved amalgamation or demerger the transferee must register from the date the Registrar of Companies issues the certificate of incorporation. The section clarifies that aggregate turnover includes supplies on behalf of principals and that goods supplied after job work are treated as supplies by the principal.