Reverse charge applicability and transitional GST treatment determine tax on supplies spanning pre GST and GST periods. Guidance clarifies that supplies invoiced or paid on or after the GST commencement are taxable under GST while payments made under prior law remain non taxable under GST; unutilized CENVAT credit may be transferred under section 140 transitional provisions; input tax credit entitlement and apportionment follow section 17 and rules 42-43; reverse charge applicability depends on GST time of supply rules and Notification No.13/2017; section 51 provides for tax deduction at source though it was not yet notified, and state wise registration is required where supplies are made from multiple States.