Each
poker site has a list of permitted and forbidden software. Of course you should only use, what is permitted. A much discussed subject over the years has been HUD, which is an abbreviation for Heads Up Display. A HUD is part of most tracking software, and it provides information about, how your opponents have played in previous
hands, where you sat with them at the table. Like how often they have put chips in voluntarely preflop (abbrevation VPIP), how often they have raised preflop (abbrevation PFR) and so on and so forth.
I currently only play on sites, that allow HUDs (
pokerstars,
888 Poker and ACR). But should I start to play on sites, that dont allow it, then the HUD would not even work there, because the company making it (PokerTracker) has no incentive to make software, that can only be used against the terms of the sites. For that reason they also comply with requirements from for instance PokerStars about, which stats are allowed to be shown, and which are not. For instance tournament stats based on stack sizes are not allowed.
There is other software like ICMizer or Equilab, which are allowed by sites but only, when you are not playing. And in those cases you should obviously remember to close those programs, before you open the poker client. The client of PokerStars send out a warning, if you forget to do this.
A lot of other software is allowed by sites but only for off-table study. And of course you should remember to close down such software, before you start playing. This typically include programs, that do advanced
equity calculations or determine Game Theory Optimal (abbrevation GTO) solutions to specific situations like preflop push/fold spots in tournaments and more.