若有指定，就會為此應用程式建立根虛擬目錄。若沒有指定，所建立的應用程式就沒有根虛擬目錄，而且在建立根虛擬目錄之前，都無法使用。

If specified, will cause the root virtual directory to be created for this application. If omitted, the application will be created without a root virtual directory and will not be usable until one is created.