Well 2000/XP/2003/Vista come with the application compatibility database installed by default. This database already contains the fixes that MS has tested and will be automatically run if the compatibility engine in the Operating System recognizes the game. The ACT can be used to see what fixes are being used on the game which makes it easier to apply fixes to say a newer version of the same game (EA games mostly.... 😉 ) or you can apply fixes to a game that is not in the database and finally you can create your own .sdb (a sort of mini database) that can be easily deployed to any computer that will automagically load the fix for the game.

With that said I haven't had to use the ACT that much in XP since the application compatibility modes in XP (win95/win98) include a couple more options in XP than they do in 2000. Also the database in XP has been updated with alot more games than 2000. (No you cannot backport the fixes easily, the only way is to load up the database in the ACT and note which fixes are being used).

To answer your question if you load up the ACT you can see what fixes you can apply for the game but there's way too many for me to list here and they wouldn't really mean anything to most people.

Well, there's one specific example that I know of: apparently Extreme-G 2 will not work with XP SP2 but requires you to download and run it with ACT.

Surely it is not unique in this respect? Doesn't ACT 4.1 contain a more up-to-date database than the one installed in XP SP2?

It's been quite awhile since I've checked out the ACT extensively but I only remember seeing one version of the ACT that included an updated copy of the database. I think it was v3 of the ACT or something like that. I think I remember checking v4 when it came out and there wasn't an sdb in there. IIRC, it really isn't a function of the ACT to update the database. In fact in that old version of the ACT the .sdb was actually an MS update package included in the ACT package.

I stumbled upon this page, which indeed suggests that all these compatibility fixes get installed automatically with new Windows updates. I wonder what was up with Extreme-G 2 then?

I guess installing ACT and manually messing with the extra compatibility options really should be a last-ditch effort.

ACT is quite useful, it's allowed me to fix issues in: Destruction Derby 2, Extreme-G2, Need For Speed Special Edition, Need For Speed 2 Special Edition and Spy Hunter. I hope it will help more games out yet...

So it is still needed, even on an otherwise up-to-date Windows machine? What gives!?

Needed in some situations yes. Needed in most cases? No. Out of 1100 Windows games I've tested mabye only a dozen need the ACT? I'd have to check. (Not counting the games already included in the default database that are automagically applied).

BTW ACT 5 is out.