- 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 pieces of the item set you choose you have equipped, compared with the number you set.
Why you might use it
Counts how many pieces of one item set (usually a tier set) you have equipped, so a rotation can follow the set bonus it relies on, such as a Warrior playing differently with the Battlegear of Wrath bonuses, or any Retail spec whose tier bonus changes a priority. Sets you are wearing are offered in the list. While item information is still loading the count is unavailable. See also Have Item Equipped.
Example
the number of Battlegear of Wrath (set 218) pieces you have equipped is greater than or equal to 3
Settings
What each setting in the condition editor means, from the game's own help.
Item Set ID - The game's item set (for example a tier set). Sets you are wearing are offered in the list; the count is taken from your equipped items' metadata and stays unavailable while any of it is still loading.
Operator - How to compare Item Set Pieces Equipped with the Pieces value. This comparison uses basic mathematical comparison operations.
Pieces - The number of pieces of that set you have equipped.