"Vehicles that are licensed under the Motor Vehicle Code as street vehicles require an ORV license only when they are being used as an ORV. A “street-licensed” ORV does not need an ORV license to use forest roads, but it will need an ORV license if it is used on designated trails or in areas designated for cross-country use."

"Note: An ORV license is not required for street-licensed vehicles used on the frozen surface of public waters or designated ORV routes."

http://www.offroad-ed.com/mi/handbook/registration.htm

Basically, if you're on a ROUTE, you don't need an ORV sticker. If you're on a TRAIL or in a SCRAMBLE AREA, you do. (All-caps words are DNR nomenclature.)