Your document class will make an instance of your Control class:
var control:Control = new Control();
(Note: use a variable name that makes sense, like control, not "test." One good reason for always using initial capital letters for Class names is that the lowercase version is then available for a variable name. Example Loader, loader).
Then it will add it to the display list:
In short, a document class represents the main timeline of your fla file. In a brand new fla file, your main timeline (aka the root) has already been added to the stage for you, behind the scenes. So anything that is added to the root is also at the same time being added to the stage. Everything must ultimately connect to the stage (speaking in a display hierarchy sense) or it is not displayed.