- 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 has the debuff you choose. With Own Debuff ticked, only a debuff you applied yourself counts.
Why you might use it
Checks whether a unit has a debuff. The most common step applies a damage over time effect or debuff when it is missing (under NOT), such as Corruption, Serpent Sting, Sunder Armor or Faerie Fire; on yourself it can check things like Weakened Soul before Power Word: Shield. Tick Own Debuff when several players can apply the same debuff, so another Warlock's Corruption does not count as yours. On Retail and WoW Forever the game hides many auras in combat, and the condition is then unavailable; see also Debuff Time Remaining and Debuff Stacks.
Example
your target has your own Corruption
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.
Own Debuff - Only consider auras that you applied yourself.
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.
Use this exact spell rank - Unticked, any rank of the spell matches: a numeric spell selection also recognizes auras with the same readable localized spell name. Ticked, only the exact spell ID you chose matches. Conditions saved before this option keep the exact ID until you untick it. Spell names already match by name. Unavailable metadata or restricted aura identity stays unavailable, including under NOT. Different spells sharing a name can match; tick the option when that distinction matters. (only on Classic Era, TBC Anniversary, Forever)