[AS3] 关于setStyle的疑惑

import flash.events.Event;
import fl.events.ComponentEvent;
import fl.events.ColorPickerEvent;
import fl.controls.RadioButtonGroup;


var tf:TextFormat = new TextFormat();
var rbGrp:RadioButtonGroup = RadioButtonGroup.getGroup("fontRbGrp");
rbGrp.addEventListener(MouseEvent.CLICK, rbHandler);
txtCp.addEventListener(ColorPickerEvent.CHANGE,cpHandler);
msgCb.addEventListener(Event.CHANGE, cbHandler);

function rbHandler(event:MouseEvent):void {
    switch(event.target.selection.name) {
        case "smallRb":
            tf.size = 14;
                     tf.underline=true;
            break;
        case "largerRb":
            tf.size = 18;
            break;
        case "largestRb":
            tf.size = 24;
            break;
        }
        aTa.setStyle("textFormat", tf);
}

function cpHandler(event:ColorPickerEvent):void {
    tf.color = event.target.selectedColor;
    aTa.setStyle("textFormat", tf);
}

function cbHandler(event:Event):void {
    aTa.text = event.target.selectedItem.label;
}


aTa.setStyle("textFormat", tf);这条代码里的textFormat是什么东西？这个参数应该是个样式表名啊．．．但是源代码里并没有出现过这个样式表．．．．．．很郁闷，看帮助时碰到的问题，弄了一晚上了搞不明白