There are a lot of sub-classes that actual vehicles are assigned to but that aren't used by the reservation system. Here are a few pulled off of some of my recent receipts:

YB (Nissan Versa...maybe elite because it had a backup camera??)
E6 (Hybrid Hyundai Sonata)
YI (Cadillac XTS)
G4 (Infiniti Q50)
E6 (Hybrid Ford Fusion)

IIRC, Y prefacing a class code means "elite"--so YF is "elite fullsize," YI is "elite premium," etc. E must mean hybrid, which I guess would technically place it between standard and fullsize, though I've never really heard of them being considered a downgrade from a fullsize (and a certificate used to upgrade a standard will get you a fullsize, not a hybrid). FWIW, hybrids are bookable and the ACRISS code is xCAH (i.e. ICAH if it's an intermediate hybrid like a Prius, FCAH if it's a fullsize hybrid like a Camry, etc.). No idea what 4 or 6 after a class code means, though.