
A "priority queue" is a type of rotation that's based on using abilities in a specific order if the ability is up or wouldn't overwrite its own effect. As you go down the list, if the ability is down or its effect is already applied, you go to the next one. It's kind of a more situational approach to a normal rotation. Historically, I've seen it used most by DoT classes and short duration buff/limited trigger buff classes with lots of different cooldowns -- Death Knights (WoW), Jedi Shadows/Sith Assassins (TOR), Necromancers (EQ and EQ2), etc.
So, hypothetically, you might have a queue that looks like...
Poison Arrow
Blood Letter
Straight Arrow IF buffed from Heavy Shot
Heavy Shot
And so you'd go down the list, using Poison Arrow if the mob isn't poisoned, moving on Blood Letter if it still is and Blood Letter is up, then to Straight Arrow if it has the buff from Heavy Shot, or Heavy Shot if nothing else applies.
I don't know if that's the best way to play Archer, but since it seemed to me that they have so many debuffs and abilities with varying cooldowns, that's how I was approaching it. I may be wrong, however -- I didn't play the class very high.
So, hypothetically, you might have a queue that looks like...
Poison Arrow
Blood Letter
Straight Arrow IF buffed from Heavy Shot
Heavy Shot
And so you'd go down the list, using Poison Arrow if the mob isn't poisoned, moving on Blood Letter if it still is and Blood Letter is up, then to Straight Arrow if it has the buff from Heavy Shot, or Heavy Shot if nothing else applies.
I don't know if that's the best way to play Archer, but since it seemed to me that they have so many debuffs and abilities with varying cooldowns, that's how I was approaching it. I may be wrong, however -- I didn't play the class very high.

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Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))