"The stickers, which Michigan coach Jim Harbaugh brought back to the helmets during his first season in 2015, used to feature a wolverine in blue and could be awarded for any number of in-game achievements. Players accumulated helmet stickers each season and would start over the next with a clean helmet.
Now, the stickers represent anything from a win — the familiar maize sticker with a wolverine but with the overall number of the victory (the first win of the season will produce a sticker with 965) — to rivalry games, to All-Big Ten honors and academic honors, and a captain will have a sticker with a “C.”"
"“It’s a great way to reward the guys. We’ve always given ours players their helmet at the end of the their career, and this provides them with a keepsake that tells the complete story of their career rather than just that final season.”"
ESPN - "Sticking to Tradition"
"The Wolverines and Buckeyes are two of 13 FBS teams that currently use helmet decal reward systems, but they do so with different philosophies. The famous buckeye leaves are awarded for team, unit or individual on-field accomplishments throughout a single season, while Michigan's stickers display a unique image for each accomplishment and accrue year-over-year."
"Much like many elements of the Michigan-Ohio State rivalry, helmet stickers trace back to Woody Hayes (Ohio State) and Bo Schembechler (Michigan), and stir debate. Ohio State wore the stickers first, in 1967, but Schembechler's teams at Miami (Ohio) began wearing tomahawk stickers on their helmets in 1965. Schembechler brought the tradition to Michigan, and both teams wore stickers for the first time in The Game in 1969, the start of The Ten Year War between the famous coaches."