When referencing content hosted outside of the repository, you can create an empty binary to act as a placeholder for the external content, specifying the URL of the external content.
Content-Type: message/external-body; access-type=URL; URL="http://example.com/external/content/file"
See Example 4 from the Container REST API PUT method documentation for more information about creating a binary that redirects to external content.
When you retrieve the binary from Fedora, it sends a redirect to that URL. This allows you to attach relevant metadata to the binary, have it contained in a repository Container, etc., but still have the content served by a separate system.