- 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 buff you choose. With Own Buff ticked, only a buff you applied yourself counts.
Why you might use it
Checks whether a unit has a buff; Buff Time Remaining and Buff Stacks look at its time left and its stacks. The everyday uses are keeping a buff up by casting it when it is missing (under NOT), such as Battle Shout, Arcane Intellect or Lightning Shield, and reacting to a proc, such as Pyroblast when you have Hot Streak. Tick Own Buff for effects several players can apply, such as checking for your own Rejuvenation on a healing target. On Retail and WoW Forever the game hides many auras in combat, and the condition is then unavailable.
Example
you have Hot Streak
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 Buff - 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)