In 2012-2014, Bohemia allowed users to transfer their retail keys to Steam. If you did this back then, your "updated" key is now a Steam CD key that permanently lives in your Steam library. That key cannot be reused by someone else.
: If the game is already installed, keys are often stored in the registry at: arma+2+reinforcements+cd+key+updated
Buying from gives you a version of the game that does not require a constant internet connection for single-player and provides a clean installer. 3. Physical Copies In 2012-2014, Bohemia allowed users to transfer their