There is overlap. Impeachment evidence would be evidence that you confront the testifying witness that disproves his testimony. Evidence that controverts the opposing side can be admitted through any witness.
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You are speaking about "distinctions without differences". Impeachment of the witness controverts the position taken by the party for whom the witness testities. Whether it is substantive will depend on the questions. Proving that some was convicted previously of a felony and is not credible impeaches them, but is not substantive.
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