I am new to this forum and work in airport ground engineering. I have a similar question which was largely addressed in a previous post by
SFI145 on 30 August 2004 quoted below
"Performance Group A
All multi-engine turbo-jet and all multi-engine turbo-prop with more than 9 seats or with MTOW > 5,700kg
Performance Group B
All propeller driven aircraft with less than 9 seats or MTOW
< 5,700kg
Performance Group C
All piston engine aircraft with more than 9 seats or MTOW > 5,700kg
Group C is not included in the ATPL or CPL theory exams.
Hope this helps"
Could SFI145 teel me where he obtained the group definitions from. I don't think his/her later comments about group D upwards being redundant are correct as they are mentioned in the latest UK Air Navigation Order/CAP 393.
electriclightfoot if you are still looking in on this hope you are closer to an answer