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The explicit percentage chance that this creature entity will be spawned.
If the pool spawns just one entity (max limit = 1 in the respective pool_template), the core selects the entity to be spawned in a two-step process: First, only the explicitly-chanced (chance > 0) entity of the pool are rolled. If this roll does not produce any entity, all the entity without explicit chance (chance = 0) are rolled with equal chance.
If the pool spawns more than one entity, the chance is ignored and all the creatures in the pool are rolled in one step with equal chance.
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