- 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 the spell you choose. Where spells have ranks, any rank of that spell matches unless you tick Use this exact spell rank.
Why you might use it
Checks whether a unit is casting one particular spell. Use it to save interrupts for the casts that matter, such as an enemy Greater Heal, Fear or Polymorph, or to answer a specific cast with Grounding Totem, Spell Reflection or Fear Ward. Where spells have ranks, a spell picked in the editor matches any rank unless you tick Use this exact spell rank. If the game cannot read the spell being cast (possible on Retail and WoW Forever) the condition is unavailable. See also Casting and Specific Spell Channeling.
Example
your target is currently casting Greater Heal
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.
The icon next to the spell name will automatically update to display the spell that is currently selected. You can drag a new spell from your spell book or action bars into this icon slot to replace your selected spell.
Spell - This is a specific spell used in this condition. You may drag this spell from your spell book or action bars into the input field or spell icon to set this value. This input field has prediction enabled, so you do not have to type the full spell name, just select from the prediction frame when it appears.
A spell picked in the editor is matched by its spell ID, so the condition works on every client language. Any rank of that spell matches unless Use this exact spell rank is ticked. Conditions saved with a spell name keep matching by that name. If the client cannot read the spell being cast, the condition is unavailable, including under NOT. (only on Classic Era, TBC Anniversary, Forever)
A spell picked in the editor is matched by its spell ID, so the condition works on every client language. Conditions saved with a spell name keep matching by that name. If the client cannot read the spell being cast, the condition is unavailable, including under NOT. (only on Classic progression, Retail)