I know you get this question all the time and I've browsed these forums with some vague answers which is not good enough for a layman like me.
If I purchase ARMA 2: Reinforcements...
- I AM able to play on ARMA 2 servers
- I am NOT able to play on ARMA 2: Operation Arrowhead servers
In order to join most ARMA 2 servers I should purchase ARMA 2: Combined Operations? ( = ARMA 2 + ARMA 2: Operation Arrowhead? )