Category is by car type
Category A - Unlimited 4 (or more)wheel (at least 2 steer) any alignment, any body, wheel driven. Category B appears to be pretty much what we would call a production car. Look at the FIA rules for more details of Category rules
Group - by engine type
Group I Blown 2 and 4 stroke otto cycle
Group II unblown 2 and 4 stroke otto cycle
Group III Blown Diesel
Group IV unblown Diesel
Group V Blown Rotary
Group VI unblown Rotary
(there are other groups for just about everything)
Class by engine size
1 - up to 250cc
2 - over 250cc up to 350cc
3 - over 350cc to 500cc
4 - over 500cc up to 750cc
5 - over 750cc up to 1100cc
6 - over 1100cc to 1500cc
7 - over 1500cc to 2000cc
8 - over 2000cc to 3000cc
9 - over 3000cc to 5000cc
10 - over 5000cc to 8000cc
11 - over 8000cc
So a Catogory A, Group II Class 4 car would be an unlimited car with a 2 or 4 stroke unblown engine over 500cc up to 750cc displacement
Notice that some of the FIA engine classes are different from the SCTA engine classes. Also the SCTA streamliner diesels combine blown and unblown while FIA does not. SCTA puts rotary engines into regular classes with piston engines and factors the engine 'size' for rotarys while FIA does not.
The info can be found in Appendix B on the FIA web site.
