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Here are a couple of interesting observations: if you assign a hero to a neutral town, they will disappear from the map when the game starts. However, if you place an NCF (Neutral Creature File) there, the hero will have creatures of level 1-2 from the NCF. This suggests that there is an algorithm that fills the creature slots, and the quantity depends on their growth rate.
However, heroes like Razaz or Laslo do not follow this algorithm due to their specializations. The specialization is tied to a specific ID, meaning it's linked to a SINGLE unit, and the bonuses will be applied to that unit and its upgrades (but upgrades of those upgrades will not receive any further bonuses).