You get a Key that helps you quickly find a free target for you
by doing a "Target Nearest Enemy" ( Usually <Tab> ) until it finds a suitable target.
This means, it only searches for Targets in the same area that <Tab> does.
For this classes it will check against all group or party members to find a free target
that nobody else in your group or party is targeting, so you can sheep/.. it:
* DRUID: Beast or Dragonkin
* MAGE: Beast or Humanoid
* PRIEST: Undead or Humanoid
* WARLOCK: Demon or Elemental
In case of the Warrior we just make sure that we don't have the target of another warrior:
* WARRIOR: Will search a target that no other Warrior in your group or raid is targeting.