Improved Hunter's Mark causes ALL PHYSICAL DAMAGE done to the target to be increased by xxx attack power. Whether it's you, your pet, a Warrior or a Rogue attacking, they get a xxx attack power bonus while the mark is on.
It's sex in battlegrounds where you can mark someone and watch the Warriors and Rogues go mental because their attacks crit for more.
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Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.