This solves a unique issue with a multisite setup and the CDN module. If you are using Origin Pull, and the origin server is set to the same domain on all multisites then the correct multisite will not be selected for things like imagecache generation. This also acts like the Stage File Proxy module; if the file doesn't exist locally it can try another server looking for it, or it can issue a redirect to that server.
If you do not use the standard files directory setup then you can create a mapping in the GUI. If the file is not found it will fast 404.