External Verses Internal Files
Flash CS4 ActionScript 3.0 Tutorial
How to Add Movie Symbols and External swf Files to the Stage with ActionScript
When you create a Movie Symbol in your project, it is added to the Library. By testing (publishing) your movie (Cmd/Return (PC - Ctrl/Enter)) a swf file is created. The symbol is included in the swf file. More symbols make for a larger swf files.
To combat the file bloating, you can place selective content external to the host swf file. External files are generally jpg, mp3 or swf files.
Using this model, the external files are brought to the stage when requested. Let's say you have an external file that is imported with a button click. If the user never clicks the button, the file is never downloaded.
For more information about external files, see topics, Import Audio from External File, Loading an External jpg or swf into a Movie Symbol, Loading an External jpg or swf with a Loader, Loading an External jpg or swf with a UILoader.
By creating Movie Symbols inside a fla file, they are included with the published swf file.
External files reside in a separate folder and are brought to the stage only when requested. By not including them in the fla Library, the host swf file is considerably smaller.
Using ActionScript, you can add a preloader so the user sees the download progress as the external files are loaded to the stage.