Short version. Yes, ACR allows HUDs and tracking software. Hand histories are saved, and the common trackers read them.
What a HUD does
A heads-up display sits over the table and shows numbers on each opponent: how often they enter pots, how often they raise, how often they fold to a bet. The software builds those numbers from hand histories, yours and everyone's you have played with.
Why it matters at ACR
Because it is allowed, the regulars use it. By your second orbit a serious player knows roughly how you play. If you are learning, that is a real disadvantage, and it is the main reason ACR's tables are harder than BetOnline's or Ignition's.
It also works in your favour. ACR is the one big room open to most of the US where your own tracker runs without workarounds.
What is not allowed
Bots, and software that gives advice while a hand is in play. ACR now runs random captchas at the tables and blocks the virtual machines that bots usually run on. See the ACR guide for the history behind those rules.
If you do not want to be tracked
Play somewhere anonymous. Ignition and Bovada hide screen names, so nobody can build a file on you. BetOnline gives every player the same basic built-in HUD, which evens things out. The softest sites page compares them.
Common questions
Does ACR Poker allow HUDs?
Yes. Tracking software and HUDs are permitted.
Are HUDs allowed at Black Chip Poker?
Yes. It is the same network with the same rules.
Which US poker sites do not allow HUDs?
Ignition and Bovada use anonymous tables, so tracking software does not work there.