Regular Production Option

A Regular Production Option (RPO) is a standardized three-character alphanumeric code used by General Motors to designate vehicle options and modifications since at least the 1950s (see Corvette C1). RPO codes could be considered vehicle "genetic codes", since the complete configuration of a GM vehicle (as it exited the factory) can be described by specifying its year, base model, and all its RPO codes.

Source: Wikipedia — Regular Production Option (CC BY-SA 4.0)