Script handle, used to identify scripts in script operations called using spawn or execVM.

When the called script is finished, the handle will contain <NULL-script>; at that point the scriptDone command will return true.
The script can be terminated by using its handle with terminate.

While the engine doesn't contain a null variable for script handles (e.g. like objNull), it can be created with by calling an empty function:
_hdlNull = 0 spawn {}

This "null handle" will return true when tested with scriptDone, and could then be used to populate an array, for example, so that any type-specific test wouldn't fail:

_hdlNull = 0 spawn {};
_handles = [_hdlNull,_hdlNull,_hdlNull];
// only element 1 contains a "real" handle
_sqlHdl1 = execVM "someScript.sqf";
_handles set [1,_sqlHdl1];
// run a type-specific command (scriptDone) against all the array elements 
_done = {scriptDone _x} count _handles;

See also: execVM, spawn, scriptDone, terminate