- 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 whether the chosen unit exists.
Why you might use it
Use this to require a target, or negate it to act when there is no target. On Classic and TBC, a marked unit such as Skull can also be checked for existence; marked-unit selectors are unavailable on Retail and Forever.
Example
your target exists
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.
Matches when the selected unit exists. A marked unit exists when that mark is known, even if the unit is currently out of reach.