Prif ddiben y Gyllideb Atodol hon yw adlewyrchu'r  newidiadau cyllidebol ers Ail Gyllideb  Atodol 2020-21 a gyhoeddwyd gan Lywodraeth Cymru ym mis Hydref 2020 ac a ailddatganwyd ym mis Rhagfyr 2020 yn unol â newidiadau i bortffolios gweinidogol.
The main purpose of this Supplementary Budget is to reflect the budgetary changes since the Second Supplementary Budget 2020-21 published by the Welsh Government in October 2020 and restated in December 2020 in line with changes to ministerial portfolios.