GST Registration: compulsory categories and procedural requirements, including timelines and advance deposit rules for casual and nonresident suppliers. Registration under GST confers legal supplier status and entitlement to input tax credit. Liability to register arises when aggregate turnover exceeds the prescribed threshold, with effective date depending on timely application. Certain classes (inter state suppliers, casual and non-resident persons, persons under reverse charge, TDS/TCS deductors, ISDs, e-commerce operators and notified suppliers) require compulsory registration irrespective of turnover. Casual and non-resident applicants must deposit advance tax and obtain registration within specified shorter timelines; suo moto registrations by authorities are temporary and subject to formal application and verification.