At some point in your editing life, you will come across Preprocessor Commands.

The BIKI page can be found here:
http://community.bistudio.com/wiki/PreProcessor_Commands

The main point about PreProcessor commands is, they are read once at the beginning of execution. This also effects the editor, meaning that if you modify a file that contains PreProcessor commands (usually a file that #include is pointing too), you'll need to re-Load the mission in the Editor for it to have an effect.

The other annoying thing about PreProcessor Commands is that errors, will usually shutdown ARMA. If you run into issues, BIKI has this to say about PreProcessor Errors:
http://community.bistudio.com/wiki/PreProcessor_Errors