Compile and execute function (sqf).
Argument is passed to the script as local variable _this.
The function is first searched for in the mission folder, then in the campaign scripts folder and finally in the global scripts folder.
execVM works fine from scripts without assigning a variable to hold the return value, but if used from activation fields of Waypoints or Triggers there must be an assignment to a variable to hold the return value or the command will fail.