What are the different kinds of files? And how do I know when to use them?

Files differ from one another in shape, material and tooth configuration. The proper selection of a file depends on the type of material being worked, the shape of the cut, and the desired surface finish. Files that remove material aggressively will leave a rough surface finish – and those that leave a fine surface finish remove very little material at once. For more information see (insert good reference like https://paulbudzik.com/tools-techniques/Files/files.html )
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