A1/B1 fuel economy oils.
A5/B5 higher specification fuel economy oils.
A3/B3 run of the mill oils for most petrol and diesels
B4 the oil spec for direct injection diesels.
C 1, 2, 3, 4. Mid and low saps oils
E 4, 5, 7 etc. HDEO for larger trucks and buses, which can usually be used in non heavy goods diesel vehicles if you do some cross checking of specs and indeed use APi specs.
Some vehicles sold in the UK and US have APi specs that relate to HDEO
My Taxi for example can use CI 4 Shell Rimula R6 LME Nd some Mobil Delvac.
You can get better info on the ACEA website but that is a generalised explanation