- 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 spell the unit specified is currently casting can be interrupted. It does not match when the unit is not casting.
Why you might use it
Checks whether the spell a unit is casting can be interrupted. Pair it with Casting or Specific Spell Casting so an interrupt such as Kick, Pummel, Counterspell or Mind Freeze only lights up when it will work, not on a cast that is protected from interrupts. On Retail and WoW Forever the game can hide whether an enemy's cast is interruptible, and the condition is then unavailable. Use Channel Interruptible for channeled spells.
Example
your target's spell is interruptible
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.