Figured some would find this interesting.
GM's mostly moving away from the GMX-XXX and GMT-XXX program codes and using a new coding system.
It consists of four letters and numbers.
1st position is the platform.
A - Alpha
D – Delta
G – Gamma
E – Epsilon
P – Premium Epsilon
Y – Corvette
L – Lambda
K – Trucks
2nd position is platform generation. For example, Alpha would be "1," Epsilon II would be "2."
3rd is vehicle type/body style.
S – Sedan
J – Hatchback
L – Long Wheel Base Sedan
U – Crossover
Y – SUV/Truck
A – Convertible
B – Coupe
And 4th is brand.
4th Position is brand:
C – Chevrolet
L – Cadillac
B – Buick
O – Opel
G – GMC
So for example, a Cadillac XTS is P1LL. P for Premium Epsilon, 1 for 1st generation of Premium Epsilon, L for LWB sedan, and L for Cadillac.
This doesn't hold true 100% of the time, there are always variations and randomness (of course, it's General Motors
).