Stacking dmg gets added to your other stacking dmg, while nonstacking dmg buffs will multiply your dmg. So polar gives plus 30% of your damage before other stacking damage buffs and nonstacking damage will multiply your damage after stacking dmg buffs. Stacking dmg adds less damage most of the time, but it can be used with armor dmg buffs, mephisto aa and the 30% dmg elix, while other nonstacking dmg multipliers can't.