Guild Wars 2

Sharpening Stone:
Bleed foes with your next five attacks.

The wiki’s description of what an Attack is:
An attack is an action intended to cause (Damage Icon) Damage on a specific target or area.

Action can be using crossfire, intending to cause damage to the enemy target. This is called an attack.
I activate sharpening stone to apply bleeding to my attacks.
After 5 attacks, the heavy golem has 5 more bleed stacks on it. Worked as intended

Piercing Arrows:
All arrow attacks pierce targets.

Arrow attack is an action that causes my weapon to fire an arrow against an enemy, making it an attack.

I trait for Piercing Arrows to make sure my Arrow Attack can’t be blocked by something else.
I activate Sharpening Stone to make my Arrows Attack inflict bleeding, for 5 Attacks.
After 5 Attacks, my target has 1 extra bleed stack on it. There were 4 other targets, which I did not want to Attack, infront of my target. Each of those 4 targets had also 1 extra bleed stack.

CONCLUSION:
Sharpening Stone works like this in practice:
Your next five hits inflict bleeding.

TESTING:
Line up 5 heavy golems in HotM and target the one all the way at the back, activate Sharpening Stone, fire once -> No sharpening stone left. The 5 golems received 1 stack each.