I think it's just kind of understood, so I don't have any real references for you. Filling in "A dog has ___ feet" requires actual facts. Or compare these two:

I think it's just kind of understood, so I don't have any real references for you. Filling in "A dog has ___ feet" requires actual facts. Or compare these two: "The city councilmen refused the demonstrators a permit because they advocated violence. It wasn't the first time the _____ had advocated violence." "The city councilmen refused the demonstrators a permit because they feared violence. It wasn't the first time the _____ had feared violence." The syntax is identical. The words are identical, except that I swapped "advocated" out for "feared". When I swap it, the ____ changes from "demonstrators" to "councilmen." Think about what kinds of reasoning and experience and knowledge it takes you to resolve which group "they" refers to in this sentence. Most blanks might be simpler and just correspond to learning english, like when the blank is "the," but learning that is a feat too. Filling in the blanks that require broader knowledge requires somehow capturing that broader knowledge.