"self-contained" means that the resulting movie file will contain all necessary media: rendered video and audio. So you can move this file to any external disk or computer without losing any part of your movie.
The minus of this approach is that it takes much longer to export (depending on the movie length) and much more disk space since the FCE media are copied into the movie file.
With a reference movie (if not self-contained) the export is fast and the movie files is small. But the media files are still in the FCE Documents folder: you'd better not to modify your FCE project before you use (e.g. burn) the exported reference movie.
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"self-contained"?
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