- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks if the unit specified is the same character or creature as the second unit you choose, such as whether your target is also your focus.
Why you might use it
Check whether two units are the same, which answers questions like "is my target attacking me?" (your target's target is you) or "is my mouseover target my focus target?". A tank can suggest Taunt or Growl when the target's target is not you, and a damage dealer can hold threat-heavy abilities when it is. On Retail and Forever, comparisons involving a chained unit such as your target's target are always hidden, and then the condition is unavailable. See also Threat.
Example
your target's target is you
Settings
What each setting in the condition editor means, from the game's own help.
Unit - The unit this condition uses as the target of comparison.