N.C.C. stands for "Noleggio Con Conducente" and is used for those cars you can rent with
the driver. Usually it is on important cars that are used for weddings or for important guests.
Its meaning is not far from "Noleggio di rimessa" as shown below. Many thanks to
Marcello Taverna for his explanation.
This tag is usually attached to buses for tourists. I don't understand exactly what "Noleggio da rimessa" means... it's probably to distinguish these buses that are rented and therefore are often in the depot ("rimessa" in Italian), from normal buses that run everyday. Thanks to Guglielmo Evangelista for this smart explanation.
The hole in the middle between the symbol of the city of Rome and the print that you can easily see in the picture is used to join this tag to the license plate with a metal wire. In the past the tag used to be over the plate.