RPO = Regular Production Order
On my G6 the sticker is in the glove box. It will have a bunch of 3 digit codes on it. Each code stands for an item installed on the car from the Factory.
On a thirdgen FBody (Camaro/Firebird)
L03 = low output 305 V8 with TBI ingection.
LB9 = Tune Port Ingected 305 V8
L98 = Tune Port 350 V8
GU5 = Axel Rear 3.23 Ratio
GU2 = Axel Rear 2.73 Ratio
FP3 = Ratio Transaxel Final Drive 2.73
NK4 = Steering Wheel Sport Leather W/Radio Controls
103 = Trim Combination, Leather Arctic White (93)
These are some examples I pulled from my Formula's decoder I got from thirdgen.org.
The RPO list is like a mini build sheet of a GM vehicle.
I do not know if the codes are all the same for different years. Wish I knew.