That is part of Chevrolet's RPO, Regular Production Option, coding system. Same as LXX, etc...
Back in 1967, Z28 was the code for the option that included a special high perf package for example. In cases where the marketing guys thought the option was pretty special, they might badge the car with it, like the LT1 engine option in the 70's made it to a hood decal.
If you Google Chevrolet info, or RPO you'll likely fin a list of hundreds of the RPO codes.