- 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 currently casting a spell. Channeled spells do not count.
Why you might use it
Checks whether a unit is casting anything. On an enemy it drives interrupt reminders, such as Kick, Pummel, Counterspell or Wind Shear whenever the target starts a cast; on yourself, NOT CASTING keeps the rotation from suggesting something in the middle of your own cast. On Retail and WoW Forever it still works when the game hides which spell is being cast. Combine it with Cast Interruptible or Specific Spell Casting, and use Channeling for channeled spells.
Example
your target is currently casting
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.