- 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 whether your specialization is the one you choose, which on older game versions means the talent tree with the most points. A specialization of another class never matches.
Why you might use it
Checks your talent specialization, most often as a switch condition so each spec gets its own rotation: Protection versus Arms for a Warrior, Holy versus Retribution for a Paladin. Retail and Classic progression (Mists) compare your active specialization; Classic Era and TBC Anniversary compare the tree with the most points, where Feral and Guardian both mean Feral Combat. A spec of another class is always false; on WoW Forever your own class's specializations cannot be told apart, and an empty or tied talent tree is unavailable, even under NOT. See also Primary Talent Tree, Talent Tree Points and Role.
Example
your specialization is Protection
Settings
What each setting in the condition editor means, from the game's own help.
Your talent specialization. Choose a class, then one of its specializations; your own class is selected first, and other classes are offered for rotations shared between characters. A specialization of another class is always false.
Retail and Mists of Pandaria Classic compare your active specialization. Classic Era, Burning Crusade and Wrath compare your primary talent tree, the one tree with the most points; Feral and Guardian both mean Feral Combat there. Forever cannot tell its specializations apart this way. No specialization yet, tied or unspent trees and unreadable values are unavailable, including under NOT. The answer is cached until your talents or specialization change.