Reverse charge mechanism in GST: recipient liable to pay tax on supplies from unregistered suppliers; registration and ITC rules apply. Reverse Charge Mechanism requires notified recipients to pay tax on supplies received from unregistered suppliers; notified classes (e.g., real estate promoters) must register irrespective of threshold. RCM tax must be paid from the e cash ledger, cannot be paid from input credit, and input tax credit on such supplies is claimable only after payment of RCM tax. Time of supply rules differ where the recipient bears RCM liability.