I was watching a HS game the other day and after a foul ball hit out of play, the umpire put another ball in play and the pitcher squatted down on the mound and began rubbing the ball back and forth on the ground like he was using a block sander. I looked at the umpire and he was dusting off home plate. I thought, that's pretty much the definition of applying a foreign substance or scuffing / doctoring the baseball. The offensive team's coach never said a word. Hmmm.
Then a few pitches later, the kid takes a handful of dirt from the ground and begins scrubbing the ball with it. This time, the umpire is taking it all in. Again, nothing. No reaction by anyone. He did this about three more times over the next two innings.
I mentioned it to the umpire after the game and he said that it wasn't a foreign substance...it was just dirt. I mentally put the palm of my hand against my forehead. A foreign substance is anything that's not part of the baseball. It's foreign to the baseball. If a ball gets enough dirt on it or gets enough scuffs and scrapes during the game, it gets taken out of play.
At any rate even if you don't understand foreign substance and you stretch this action to perhaps removing some of the gloss off the ball, he's still applying scuffs and scrapes to the ball! In his mind, he was probably thinking that it's no worse than when the ball is hit along the ground or scrapes down the side of a chain link fence, etc. The difference is that it can't be intentionally done.
More proof that you never know what you are going to see at the ball park.