What are special variables? These variables have special meaning to the engine. They sometimes are also called reserved variables. Private special variables have private scope in relation to the function they have been assigned to and may already contain some data when function is called.
Most common private special variables you probably already know are _this which is used to pass parameters to scripts, _x which is used in forEach and count loops and _forEachIndex, used also with forEach loops. Other private special variables are less common, nevertheless they are special.
It is good practice to make sure special variables names do not clash with user made private variables names. This is why special variables are highlighted differently in my syntax highlighter to alert the user. The list below contains all of special variables I could find. If you know about other private looking special variables used in ArmA, please share.