The specifications should always have bounds, and if there is a bound for the coefficient of friction, then it is because that is what is desirable for the specific function. For example, sometimes you wouldn’t want there to be that little amount of friction in a product. However some bounds you can theoretically say would just make the product better and better if there didn’t exist a higher or lower bound, but then the cost could possibly be the reasoning behind that. For example, to make a product better may just make it more expensive when not necessary.