2 secs with Google would show you it's a Commercial Driver's License. Used for Commercial Vehicles which have a gross weight of over 26k lbs, or required to tow a trailer over 10k lbs GVR. But I'm sure you already knew that. Weight stations are for weighing vehicles that qualify as CMV's and / or that are subject to the FMCSR's, hence the 3 little red lights in the back of them, but again, you knew that.
For those that (actually) didn't know, there ya go.