First off, welcome to BB.
What they're probably talking about is a balance hole, which usually is only put in to a) make the ball legal as far as balance is concerned or b) fine tuning the reaction when it's thrown. If the ball driller weighed the ball out to make sure it was within one ounce left vs. right (after it was drilled) and it was heavy on one side - he would need to put in the weight hole - and yes it will be open to the world. However it's not the end of the world.
I am a bit . . . . concerned that they're already talking weight hole when they've not put any finger/thumb holes in the ball already. Did they ask you / watch you as you normally bowl to get an idea about how you play? That would help them to better figure out how to drill up the ball to better perform for you.
Let us know what happens.
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