They are not attacks, they are separate automatic hits and as such don't use the weapon's stats. Impact hits are a result of the mass of the unit being thrown at the enemy, not the weapons being used against them. An ogre's belly doesn't have an AP value.
I'm not entirely sure about how the Impact values are combining. My understanding is that if one unit for example the Dread Knights  from Ghostly Undead army has Impact(1) as a special rule, it stays Impact(1) no matter if the unit has 5 models, 10 if the unit is doubled or just 1 because it's the only survivor, it stays Impact(1). However, if the unit has impact Weapons, for example the Glaive Stalkers  which spears has Impact(1), then the impact is multiplied by the number of models in the unit having that weapon. And it the unit has both impact from weapons and from special rules, these are adding up. Which becomes even more complex when the hero has impact in Special Rules, then I guess I need to add the Impact from special rules of the hero to the Impact from special rules of the infantry unit, then add the impact of weapons multiplied by the number of models having that weapon to know the real Impact value. Am I understanding it right?
Impact is a rule that applies to the model, not the unit. This means that a unit of 5 models with Impact(1) deals 5 hits. Heroes work the same way, so if a hero with Impact(3) joins a unit of 5 models with Impact(1), then you deal a total of 8 hits.
Additional questions about impact: (sorry for necro)
Last friday, playing sith a fiend some odd interactions made us stall until we could decide how to resolve impacts:
A-. Are impact hits considered melee? If so, are those sounds taken in account to check melee results? (I thought they happened beforehand, giving the charging unit an advantage in the upcoming fight, my friend stated it was melee and the wounds had to sum up the rest)
B-. If it is melee, do these hits benefit from my Cursed Lantern: The hero and his unit get AP(+1) in melee?, note it doesnt say with melee attacks (As he insisted on impacts were melee I then said my impacts would have AP. In the end I didnt wnt to go on with the discussion and didnt aply it, and I didn't want to use the base AP(1) from glaives as AP(2) with the impacts, just AP(1) )
Thanks for your help
Last Edit: Aug 9, 2020 8:53:38 GMT by txousman: Format for easy reading
As far as I know (and how I play it) impact is not melee. It's an automatic hit takin place before any melee action. Therefore it isn't affected by any melee buffs or debuffs. I would even say that if a unit wipes out another unit just by impacts and without fighting melee it wouldn't count as fatigued.