PokerTracker

What is the purpose of this stat?
How would I use it in my HUD?

Is that a custom stat? You should be able to see how it is defined by going to Configure > Statistics, but I'm not sure how a live effective stack would work, since we define the effective stack at the player's first action in a hand (which isn't possible in a live stat). I guess it would be the size of the shortest stack at the table, as that is the only thing we can know before the hand starts, but without seeing the stat I can't say for sure.