Application tag in Flex

The tag defines the Application container that is always the root tag of a Flex application. In addition to being the root tag of a Flex application, the tag represents an Application container. A container is a user-interface component that contains other components and has built-in layout rules for positioning its child components. By default, an Application container lays out its children vertically from top to bottom. We can write an MXML application in a single file or in multiple files. We typically define a main file that contains the tag. From within our main file, we can then reference additional files written in MXML, ActionScript, or a combination of the two languages.
In Flex, we can divide our application into separate MXML files and ActionScript files, where each file corresponds to a different module. By dividing our application into modules we provide many benefits, including the following:
Ease of development: Different developers or development groups can develop and debug
modules independently of each other.
Reusability: We can reuse modules in different application so that we do not have to
duplicate our work.
Maintainability: We can isolate and debug errors faster than we could if we application was developed in a single file.