actually, this tier system does not exist...only different services/schemes exist in the Ministry and SB...

if you wanna sort them, this is my 2cts

Tier 1: Administrative

Tier 2: MFA/Foreign Service, MinLaw/Legal Service, MTI/Economist Service, MTI/Statistician, MOH/Medical Services

Tier 3: MAS, Govtech, MCI/Cybersecurity Professional Scheme, MOE/Education Service, Accounting Profession, Auditing Service

Tier 4: Management Executive Scheme (SB scheme usually map over)

tier 1 - ~5% of govt - PS/DS/CE/DCE/ED/MD etc
tier 2 - ~10% of govt - very very niche skillset
tier 3 - ~25% of govt - very nich skillset
tier 4 - ~60% of govt - Generalist and some nich skillset

EDB and ESG do more on the marketing and by meeting KPI provides the staff more corp bonus la... the work-life balance is damn bad..

MHA/Home Affairs Services, Mindef/Uniform - technically not consider part of govt.
all this info can be found in the budget websites