A slug is a round piece of vinyl or urethane that is put into a ball. Instead of drilling your thumb hole directly into the ball, you can just drill it into a slug. The slug gives a more consistent feel (core and coverstock feel different on a ball). You can also change out a slug without plugging the hole. If the hole doesn't have to right pitch or is too big or something else, you can change it for 10 bucks and 5 minutes instead of 2 days and $30 to plug it. Also, ask to get an oval thumbhole instead of a circular one. As for grip inserts, those are so you can get more lift on the ball.
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