Supply under GST defines the taxable event and allocates levy, exemptions and reverse charge obligations accordingly. Article 246A (read with Article 269A) vests concurrent GST powers in Parliament and States, with integrated tax for inter State trade. The taxable event is supply of goods or services, including specified no consideration supplies; governments may notify activities as supply or exempt them. Composite supply is taxed as its principal supply, while mixed supply attracts the highest rate among its components. The reverse charge mechanism shifts tax liability to recipients for notified supplies, with a specified list of goods under reverse charge and temporary abeyance for supplies from unregistered persons. Casual taxable persons must pay estimated net tax after due input tax credit. Electronic commerce operators can be made liable to pay tax for specified services supplied through them.