taxicabs are required a sign on the side stating location of registration and such... allowed painted, sticker, etc by local police and inspectors as long as visible from certain distance... Even allowed magnetic sign in certain places rather than permanent...
In England taxicabs have same requirement for signs, but make it an actual thing bolted to license plate area rather than permanent to bumper or back of vehicle, so it can be changed yearly with expiration dates, etc...
So, I guess definition of sign is in the eye of the local AHJ?