Document Class and Loading classes into this class

programming flash
1

Hi there, im trying to move to AS3 …
So I came across this code, where the fla has a document class of Main.as
This loads in … Ok. But what I dont get inside this code, is that it manages to load in Secondary.as without a form of reference I am familiar with :::: it must be easy stuff, but look at this…(Main.as)

package
{
    import flash.display.MovieClip;
        
    public class Main extends MovieClip
    {
        private var ac:Secondary;
                
        public function main()
        {
        ac = newSecondary ();
        addChild(ac);
        }
    }
}

Seconday.as is as follows (it doesnt do anything)

package 
{
    import flash.display.MovieClip;
    
    public class Secondary extends MovieClip
    {
        public function Secondary()
        {
            addEventListener(Event.ADDED_TO_STAGE, initThis);
        }
        private function initThis (evt:Event)
        {
            test = new Sprite();
            this.addChild(test);
        }
    }
}

any explanations would be tip top…