Nella documentazione presente sul sito trovi degli esempi che possono aiutarti a capire meglio

callback function to be called when finished (function or object with scope, func and args or string)

function as callback

function onEnd(){
trace("onEnd");
}
my_mc.tween("_x",100,1,"linear",0,onEnd);

// scope of function is my_mc._parent

object as callback

You can pass as callback object with properties

func - function to be called when tween is finished
scope - scope of function (this in called function)
args - array of arguments passed to function

updfunc - reference to function to be called on every update
updscope - scope of update function (this object)
updargs - array of arguments passed to update function

startfunc - reference to function to be called on start of tween
startscope - scope of start function (this object)
startargs - array of arguments passed to start function

* internal mechanism is: func.apply(scope,args)

// on _root
game={};
game.players = ["john","steve"];
game.showScore = function(id, score){
trace("(this==_root.game) is "+(this==_root.game));
trace(this.players[id] + " has " + score + " points");
}

// somewhere in nested movieclip
var callback = {scope: _root.game, func: _root.game.showScore, args:[1,39]};
my_mc.tween("_x",100,1,"linear",0,callback);
/* or in 1 line:
my_mc.tween("_x",100,1,"linear",0,{scope: _root.game, func: _root.game.showScore, args:[1,39]});
*/

//output after finishing tween:

(this==_root.game) is true
steve has 39 points

string as callback

callbacks can be too defined as strings

my_mc.tween("_x",100,1,"linear",0,'_root.gotoAndPl ay(8)');

it is very problematic determine type of primitive parameters(number, string, boolean), in this case is 8 string

for save type of passed argument use references:

function callMe(my_obj, my_nr, my_bool) {
trace(my_obj +">> typeof(my_obj) is "+ typeof(my_obj));
trace(my_nr +">> typeof(my_nr) is "+ typeof(my_nr));
trace(my_bool +">> typeof(my_bool) is "+ typeof(my_bool));
}

test_obj = {name: "test", id: 10};
test_bool = true;
test_nr = 99;

my_mc.tween("_x",100,1,"linear",0,'_root.callMe(te st_obj,test_nr,test_bool)');

Do not add spaces between argumets

Lo scope come puoi vedere dagli esempi è praticamente l'oggetto che contiene la funzione (come puoi vedere ad esempio nella parte di esempio

game={};
game.players = ["john","steve"];
game.showScore = function(id, score){
trace("(this==_root.game) is "+(this==_root.game));
trace(this.players[id] + " has " + score + " points");
}

// somewhere in nested movieclip
var callback = {scope: _root.game[...]

La funzione "showScore" è contenuta nell'oggetto "game", e quindi come scope tu indichi il percorso di tale oggetto, che è appunto "_root.game" (in quanto si trova sulla timeline principale)