They're all the 3 digit RPO codes (or whatever the are, if not RPO). There's about a bazillion of them within GM. I know that from a little bit of experience working on a GM production floor.
I guess I thought I was always thinking of it as nine-cee-one and that that wasn't right, but apparently that's what everyone else calls them. Now that I think of it, GM would definitely have used "Cee". The only people who would've called it "charlie" are the folks who drove them, and I'm sure they didn't even know what 9C1 meant (most of them).