Registration requirement: timely GST registration and single-registration rule govern liability, eligibility, and unique identity issuance. Registration requires application within the prescribed short period by persons liable to register, with casual and non-resident taxable persons to apply before commencing business; supplies from territorial waters require registration in the State of the nearest baseline. A single registration is granted with separate registrations permitted for distinct business verticals. PAN is ordinarily required for eligibility, with TAN accepted for deductors and alternative documents for non-residents. The proper officer may register non-compliant persons; certain notified international organizations may receive a Unique Identity Number. Issuance is subject to prescribed verification, certificate formality, and deemed grant if no deficiency is communicated within the verification period.