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"Reinforcement" - I suspect that this ability works similarly to "Summon Gate" and "Necromancy," meaning the number of reinforcements depends on the total HP of the army. The percentages could very well match the percentages of the first three levels of the abilities mentioned above :).
**XEL**, what's the deal with the fourth mission of Haven and the second mission of Sanctuary?
By the way, "Buka" isn't entirely to blame - the translation is clearly taken from the beta translation in some places. Although, if "Buka" translated the beta as well... In general, yes, the translation leaves much to be desired; I would describe it as "high-quality pirated" rather than "licensed," if not for the voice acting. But even the voice acting isn't very expressive. In "Heroes of Might and Magic V," the situation with the licensed translation was much better, in my opinion.