I believe it increases the accuracy stat as it would with Tussle Technique for attack speed. So it's +10% accuracy on everything.
Bear in mind that in the future, Accurate Markmanship will be reduced to Lv3 (+6% Acc, +6% Range, +5% mana gain) as a nerf I suppose, or to be equal with regular 2nd job skill tree passives. I suggest you live with Lv3 so you do not have to re-adapt to the new build. Though it may not affect you much, and the change is only likely to happen after many months, plus, you get to use 2 SP in another skill to save some time.