In iRBF Armour Defense weapon stat used to boost both your total physical and magical defense - in difference to the skill protection, which also gave Armour Defense % but only boosted the white stats of your armours.
Since they now make a distinction in RBF:Origin; there are two options:
- Either AD works still as above and makes does no difference in having AD or PD %, unless you want your m.def boosted as well.
- They changed it and it now boosts only armour stats, whereas p.def boosts your actual physical defense. Either case, you will want either or the both of them for tanking purposes (PvE mainly).
I haven't tested this yet.
As for skill defense, this will matter mostly in PvP, or later stage endgame pvE. Since in order to have skills hit minimum dmg, you would need an extreme amount of defense your opponent cannot 'break' with his attack. Unless you're full vit, with enough boosters, this is nigh impossible.
Regardless of that, (some) skills also deal magical damage or damage that just ignores defense in general. In that case, the only way to lower damage is by using damage drop (IM) or skill defense %.
Not quite sure if it's accumulative with said damage drop, or if it's a different calculation.
Of course, I could be completely wrong - in which case this will be buried under a text wall of shame and can be disregarded as complete nonsense.