I have wondered this myself, and I have discovered very little. Right now I think (I'm not sure) that they are chunks the AI will not path through. For example, if you had a lake shaped like an infinity symbol and your base was at the bottom, the top middle might be considered bad attack chunks because biters cannot go through them to get to your base. In your case, biters may be so dense it would be quicker to go around.
Again, I'm not sure about this and would like to hear the answer from a more knowledgeable person.
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