Why are some sprockets marked as "track use only"?

Sprockets are marked with "track use only" whenever the chain size of this sprocket is narrower than stock.

Narrower chains ofcourse mean less weight, on both the chain and the sprockets. However it is not without a disadvantage.

Narrower chains cause increased pressure on the sprocket tooth. Because of this we cannot garuantee sprockets will last as long as would normally be expected. For this reason we do not recommend use of narrower sprockets on road bikes.