Explain the "Revisional Authority" with its powers as per the provisions of CGST Act, 2017. Also List out three instances where authority cannot revise the order.

Section 2(99) defines "Revisional Authority" as an authority appointed or authorized under the CGST Act for revision of decision or orders referred to in section 108. Section 108 of the Act authorizes such "revisional authority" to call for and examine any order passed by his subordinates and in case he considers the order of the lower authority to be erroneous in so far as it is prejudicial to revenue and is illegal or improper or has not taken into account certain material facts, whether available at the time of issuance of the said order or not or in consequence of an observation by the Comptroller and Auditor General of India, he may, if necessary, can revise the order after giving opportunity of being heard to the notice. The "revisional authority" can also stay the operation of any order passed by his subordinates pending such revision. The 'Revisional Authority" shall not revise any order if- The order has been subject to an appeal under section 107 or under section 112 or under section 117 or under section 118; or The period specified under section 107(2) has not yet expired or more than three years have expired after the passing of the decision or order sought to be revised. The order has already been taken up for revision under this section at any earlier stage. The order is a revisional order.