PokerTracker 3 has a built-in hud system. It is a complete tracker and hud package and no additional software is required.
PokerTracker 2 was only tracker software and does not have a hud. Thus PokerAce Hud was created to interact with PokerTracker 2.
It is not possible to run a hud without a tracker because the tracker provides the statistics for the hud. This is why modern tracking software has the hud portion built-in.
PokerTracker 2, Poker Ace Hud, and PokerTracker 3 are all on the approved software list for PokerStars. You can read more about this on PokerStars Third Party Tools and Services FAQ