- 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 channeling can be interrupted. It does not match when the unit is not channeling.
Why you might use it
Checks whether the spell a unit is channeling can be interrupted. Pair it with Channeling so an interrupt such as Kick, Pummel, Counterspell or Wind Shear only lights up for a channel it can actually stop. On Retail and WoW Forever the game can hide whether an enemy's channel is interruptible, and the condition is then unavailable. Use Cast Interruptible for ordinary casts.
Example
your target's channeled 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.