Service stickers are most often seen on the top left corner of the interior windshield. They act as a reminder to the driver for important maintenance and service dates and appointments. Most important though is that they do not obstruct driver visibility - so they are most often printed on a clear or translucent white substrate.
Another important function of a service sticker is that it can be easily removed and even replaced - such as oil change stickers that are updated with your next suggested oil change mileage. This function is best performed by static cling vinyl or polyester. Service stickers also often have areas blocked out (sometimes in white ink) so that these dates are easily read if the decal is printed on a clear background.
Service stickers are never very large and are most often in a rounded corner rectangle shape. Rounded corners are better for static cling decals as squared corners can tend to lift off the windshield.
Service stickers are used primarily by the maintenance division of the dealership or independent service stations and oil change facilities.