A CDL is a commercial drivers license. It's what qualifies a person to drive a commercial vehicle, like it says. According to the Internet it can also stand for "Call of Duty League" but most people I know with CDL decals on their cars are advertising that they're truck drivers. You might be getting it mixed up with a CCW decal. I've never seen one but apparently they exist, and it looks like they're done in the same black & white format.