X is for Xtreme
"The mammoth rear wing of the 2018 Chevrolet Camaro ZL1 1LE sports prominent "X" decals. It's to signify, say Chevrolet engineers, that this is the most extreme interpretation of the sixth-generation pony car. For enthusiasts who measure a car's worth by how well it romps around a racetrack, you may agree that those "X" stickers are warranted given the ZL1 1LE's Nürburgring lap time of 7:16.04."
https://www.cnet.com/roadshow/review...o-zl1-preview/