Sec. 42. 43

https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://studycafe.in/2017/10/section-43-cgst-act-2017-matching-reversal-reclaim-reduction-output-tax-liability.H T M L&ved=2ahUKEwijxuW35s3aAhUT4o8KHWbwC2QQFjAMegQIBRAB&usg=AOvVaw2ro4F9iAeawuUxAUy6z1-J

In short

section 42 deales with recepient taken input is match with supplier output, if not the both are intimated by the govt. to do rectified , or recejpent has to reverse the & add to his ouput liability

Section 43 deales in supplier output by credit note issued to receipient, if not match the supplier has to reverse the same by inreasing his ouput