GST registration: liability triggers mandatory application, single registration principle, PAN requirement and Unique Identity Number provisions. Persons liable to registration must apply within the prescribed period, with special advance-application rules for casual and non-resident taxable persons and a territorial-waters rule. A single registration is generally granted, though separate registrations may be allowed for distinct business verticals; voluntary registration is permitted. A Permanent Account Number is required for grant of registration, with a TDS account number as an alternative for deductors and prescribed alternate documents for non-residents. The proper officer may register noncompliant persons; certain international and notified entities may receive a Unique Identity Number. Grant or rejection follows prescribed verification and time limits, and a certificate of registration is issued in prescribed form.