I am a little confused about multiple wounds. With, say, a model that caused multiple wounds X3, wound it mean that against standard one-wound R and F models one unsaved wound would cause three casualties, or is it only to help you kill big things?
A single attack can never do more wounds to a model than the wounds characteristic that the model has. So against 1 wound RNF the multiple wounds will always count as a single wound. Against a model that has 2 wounds d3 wounds could do either 1 or 2 wounds (if you roll 3 wounds it would only count as 2 because you can't do more than the wounds characteristic).
That is only if you are fighting a multiwound model, it will have no effect if you are fighting a 1 wound champion
Pretty sure thats irrelevant as for the purpose if overkill multi wound counts regardless of models starting wounds.
Incorrect, a single attack can never score more wounds that what is on the models profile. The overkill rule refers to multiple Attacks with the multiple wound special rule in on a character with more than one wound on its profile. Example. a hero with 5 attacks using a multiple wound 2 weapon is fighting in a challange with a 1 wound champion. lets say 3 of the attacks wound, the the hero scores 3 points for combat resolution, 1 for the wound killing the champion and 2 for overkill, the multiple would rule does not come in effect since the champion only has one wound. same hero in a challenge with a 2 wound hero, 3 attacks wound again, this time the hero gets a total of 6 to combat resolution, 2 for the wounds caused, and 4 for overkill, here the multiple wound rule comes in to play since the model has 2 wounds on its profile.
As per the rules on page 103, 2nd para, under Overkill: "Note that this is an exception to the rule stating that a model can only suffer as many wounds as it has on its profile" A little further down in the same paragraph it explicitly lists attacks causing multiple wounds as counting toward the total number of wounds caused. That being the case, multi-wound attacks count in challenges regardless of the number of wounds the opponent has. The only effect fighting a champ has is in limiting the maximum overkill to the champ's wounds +5 = 6 (unless fighting a monstrous inf/monstrous cav champ with 3 wounds, in which case 8 would be the max for overkill.)
I do not agree. The 2 rules work fine together, this part "Note that this is an exception to the rule stating that a model can only suffer as many wounds as it has on its profile" simply states that you count all the wounds caused to the model instead of stopping at the amount the model has, it does not override the rule stating that you cannot cause more wounds on a single attack than the model has on its profile, therefore the 1 wound maximum pr. attack when fighting a one wound models is still correct.
Stonecutter didn't quote the next sentence in the rule: "This time you need to add up all of the wounds inflicted on the victim, even those from a weapon causing multiple wounds, or by repeated Killing Blows, etc." So, in a challenge against a Champion the Multiple Wounds does apply, up to a maximum of 5+W.