NB2 marks a vehicle as built to California emissions specifications. On 1982–1992 third-gen F-body cars NB2 typically pairs with a different exhaust gas recirculation calibration, sometimes a different catalytic converter and oxygen sensor configuration, and occasionally — on early TPI cars — a slightly de-rated horsepower number. On the SPID label the NB2 callout sits in the option block alongside the engine RPO.
For collectors and authenticity sticklers, NB2 is one of the codes that pins a car to a specific original-sale state (or to a specific compliance package even if first-sold elsewhere). Plenum reports the callout and links to EPA emission certification records where available.