- 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 unit specified has been recorded by the MobInfo2 addon as immune to the spell school you choose. With Partial ticked, a mob that was only sometimes immune also counts, and a mob you have never fought never matches.
Why you might use it
Skip spells a mob is known to be immune to, for example Fire spells against fire elementals, so a Mage can fall back from Fireball to Frostbolt automatically. It needs the MobInfo2 addon and relies on what MobInfo2 recorded in earlier fights, so it can be wrong and is always false until you have fought that kind of mob; check Partial to also count mobs that are only sometimes immune. On Retail and Forever the game often hides an NPC's name from addons, and then the condition is unavailable. See also Resistant.
Example
your target is immune to Fire
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.
Spell School - The school of magic resistance to check.
Partial - Allow partial immunity (ie. the mob is only sometimes immune to this type of magic) to count.
This condition uses data from the MobInfo2 addon. As such it relies on previous interactions with the type of mob in question, which may be inaccurate. Additionally, until you encounter the mob at least once, this condition will always be false.