A- SUBCOMPACT (Kia Rio, Hyundai Accent)
A4- Chrysler Crossfire
B-COMPACT (Chevy Cobalt, Ford Focus, Kia Spectra)
YB- SPECIALTY COMPACT (Ford Focus, Chevy Aveo, Hyundai Elantra, Honda Civic, Honda Fit, Kia Spectra, Mazda 3)
B4- Ford Edge
C- MIDSIZE (Toyota Corolla, Kia Optima, Mazda 6)
YC- SPECIALTY MIDSIZE (Mazda 5, Mazda 6, Chevy HHR, Chrysler PTCruiser, Mercury Milan, Volkswagen Jetta)
C4-SPECIALTY MIDSIZE FUN GROUP (Hyundai Sonata)
D- STANDARD (Chevy Malibu, Pontiac G6)
YD- SPECIALTY STANDARD (Pontiac G6 GT)
D4- Ford Mustang
E4- Ford Fusion
F- FULLSIZE (Ford Taurus, Buick Lacrosse, Chevy Impala, Pontiac Grand Prix)
YF- SPECIALTY FULLSIZE (Ford Taurus, Buick Lacrosse, Chevy Impala, Ford 500, Pontiac Grand Prix GXP, Mercury Montego, Dodge Charger)
F6- Toyota Camry
G- PREMIUM (Mercury Grand Marquis, Ford Crown Victoria)
G4-Infiniti G35
H- PRESTIGE PREMIUM (Toyota Avalon, Hyundai Azera, Kia Amanti)
YH-SPECIALTY PRESTIGE PREMIUM (Toyota Avalon, Hyundai Azera, Kia Amanti)
I- LUXURY (Lincoln Towncar, Cadillac DTS)
YI- SPECIALTY LUXURY (Lincoln Towncar, Cadillac DTS)
L- STANDARD SUV (Chevy Trailblazer, Toyota Highlander)
YL- SPECIALTY STANDARD SUV (Toyota 4Runner, Toyota Highlander, Chevy Trailblazer, Jeep Commander)
YP- Ford Freestyle
Q- MIDSIZE SUV (Ford Escape, Mercury Mariner, Subaru Outback, Subaru Forester, Jeep Liberty, Chrysler Pacifica)
YQ- SPECIALTY MIDSIVE SUV (Ford Escape, Mercury Mariner, Subaru Outback, Subaru Forester, Jeep Liberty, Chrysler Pacifica)
Q4- SPECIALTY MIDSIZE SUV (Ford Escape, Mercury Mariner, Subaru Outback, Subaru Forester, Jeep Liberty, Chrysler Pacifica)
Q6- SPECIALTY MIDSIZE SUV (Ford Escape, Mercury Mariner, Subaru Outback, Subaru Forester, Jeep Liberty, Chrysler Pacifica)
R- MINIVAN (Dodge Caravan, Chrysler Town and Country, Kia Entourage, Ford Freestar, Toyota Sienna, Chevy Uplander)
YR- (Dodge Caravan, Chrysler Town and Country, Kia Entourage, Ford Freestar, Toyota Sienna, Chevy Uplander)
T- FULLSIZE SUV (Dodge Durango, Ford Expedition, Chevy Suburban, Chevy Tahoe)
U- CONVERTIBLE (Ford Mustang Conv., Chrysler Sebring Conv., Volkswagen Beetle Conv.)
V- Jeep Wrangler Convertible

Any car class that has a Y before it or a number (i.e. 4, 6) after it means the car has more options (meaning its not just a base model). These options include sunroof, cruise control, leather seats, etc.

If I can think of anything else I will post it.