Where you have paid an amount of tax according to a set of rates and bands, but those rates and bands are subsequently rejected by the National Assembly for Wales (section 63A TCMA)
os ydych wedi talu swm o dreth yn unol â set o gyfraddau a bandiau, ond mae'r cyfraddau a'r bandiau hynny’n cael eu gwrthod wedi hynny gan Gynulliad Cenedlaethol Cymru (adran 63A DCRhT)