Registration requirement: persons liable must apply within prescribed time, with special pre-commencement rule for casual or non resident taxpayers. The provision requires persons liable to GST registration to apply within the prescribed period, with pre-commencement application rules for casual and non-resident taxable persons, and mandates registration in the coastal State for supplies from territorial waters. It provides for single registration with separate registrations for distinct business verticals, permits voluntary registration, treats multiple registrations or interstate establishments as distinct persons, requires a Permanent Account Number (or specified alternatives) for eligibility, allows proper officers to register defaulters, and establishes Unique Identity Numbers and prescribed verification, issuance and deeming mechanisms.