in the ArmA 2 installation folder are 2 readme files, readme_fxaa and, readme_smaa.
read those.
PPAA is post processing anti-aliasing.
smaa and fxaa are new and improved ways to anti-alias the image in games, with less impact on framerates than older methods of anti-aliasing.
you can also check this page for the explanation of the settings in the ArmA2.cfg file:
http://community.bistudio.com/wiki/arma2.cfgAToC is Alpha-To-Coverage Anti-Aliasing.
i found this page that explains AToC and gives some examples of the different settings:
http://www.10-78.com/forums/showthread.php?t=27601PPAA and AToC can improve/alter the quality of the image in ArmA 2/OA.