Central Office Production Order.
If you wanted a fleet [at least 50 I believe] of cars with non-normative options or equipment a request was placed with Chevy's Central Office in Detroit. If Engineering approved it a number was assigned and the cars got built. Mostly used for police, fire, commercial vehicles. Non-RPO paint also used this process.
Most famously used for back-door hot rods such as the ZL-1 & L-72 Camaro. Someone who would know told me GM management was quite upset when they learned the process was used to circumvent corporate policy regarding engine size limitations for certain car lines.