onReapplyDuplicateType refers to what happens when a buff is applied to a target a second time. Say the buff has a stacking limit of 1, and it has a finish condition of time elapsed; in this case using PrioritizeNewBuffs would be good as the "new" buff, which has a "new" timer, would be better to apply to the target. However, when dealing with say a passive ability that only has out of range as a finish condition, using PrioritizeOldBuffs would be better as this uses up less CPU resources and as reapplying the buff ultimately serves no purpose.

buffStackingLimitType refers to the "owner" of the original caster. For example, with a stacking limit of one and a buffStackingLimitType of ForAllPlayers, a phase gate buff will only work for a single player, if there are multiple phase gates in a gravity well. However, with a buffStackingLimitType of PerPlayer means that each player has a stacking limit of 1, i.e. everyone can have a phase gate buff on that gravity well.

From what I can tell buffExclusivityForAIType goes hand in hand with buffStackingLimitType, namely if you have a buffStackingLimitType of PerPlayer buffExclusivityForAIType of ExclusivePerPlayer. However, for buffs that applies to all players rather than being focused on per player, it seems that ExclusiveForAllPlayers and NotExclusive effectively do the same thing, confusingly.

Yes, that is correct; with how phase gates are set up, only A or B will have the phase end point. The phase gate ability is also rather terribly done; if a phase gate buff is "trumped" by a new one, the other phase gate is useless as it will never properly reapply the phase end point buff again.

That's not how it works. It means that say Ship 1 and Ship 2 have an ability that does -50% acceleration to an enemy ship. If the stacking limit is 2, and both ships can cast the ability infinitely, with ForAllPlayers the enemy ship will experience -100% acceleration, as the buff can only stack two times. However, if the buff is set to PerPlayer, the ship will experience -200% acceleration, as both Ship 1 and Ship 2 each cast the ability two times, because each player can apply the buff 2 times.

The percentage bit was purely an example. Simply put, in Sins, -100% does not mean zero.

Thanks a lot. Are there other 'special cases'? I think the damageAsDamageTarget must be one, but I have no mind about other.

No, I mean that 100% of this parameter leads to invincible. I wonder what are other cases that 100% or -100% means zero. Planet Population reduction may be one, as you say. I am interested in others.