- 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 how many stacks the debuff you choose has on the unit specified, compared with the number you set. An absent debuff does not match.
Why you might use it
Compares the stacks of a debuff. Use it to build a stacking debuff up to its maximum, such as five Sunder Armor or Fire Vulnerability stacks in Classic, or to taunt-swap when a boss debuff stacks too high on you. A missing debuff does not satisfy it; on Retail and WoW Forever the stack count can be hidden in combat, and the condition is then unavailable. See also Debuff Present and Buff Stacks.
Example
the number of stacks of Sunder Armor on your target is less than 5
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)
Operator - How to compare Debuff Stacks with the Stacks value. This comparison uses basic mathematical comparison operations.
Stacks - Compare the application count of a matching aura, including charges such as Earth Shield. An absent aura does not satisfy this condition; an unreadable count is unavailable.